[Objectives] To enlarge the population size and distribution area of Crested Ibis (Nipponia nippon), 20 individuals were introduced from Yangxian to establish an acclimatizing population in Beidaihe in July 2018. The reproduction of the Crested Ibis has always been the focus of the research for its conservation. Since the introduction of the Crested Ibis into Beidaihe, we have been continuously monitoring the reproduction of this population, and discovered the phenomenon of extramarital mating and intraspecific parasitism for the first time. [Methods] During the breeding period of 2020, three high-definition cameras installed in the cage house were used to observe the Crested Ibis and relevant data were recorded. A nest with laid eggs was recorded as a breeding nest, and when we found that at least one chick fledged out of the nest, it was recorded as a successful nest. Accordingly, the success rate of nesting is defined as the ratio of the number of successful nests to the total number of nests, the reproductive success rate is defined as the ratio of the number of fledglings to the number of eggs laid, and the reproductive productivity is defined as the number of fledglings per nest. The breeding data of Beidaihe and Yangxian in 2020 were analyzed by Excel and SPSS19.0 software. The independent sample t-test and the Mann-Whitney U test were used for comparison, according to if the data were in line with the normal distribution. [Results] During the breeding period in 2020 in Beidaihe, there were altogether 22 Crested Ibis, including 17 adults, with a female-to-male ratio of 1.1. The actual breeding density was 37.8/hm2, the nest density was 40.0/hm2, and the inner diameter of the nest basket was 50cm. Of the 8 pairs of Crested Ibis, 6 pairs were successful in breeding, with a nesting success rate of 75%; a total of 33 eggs were laid, with an average clutch size of 4.1 ± 1.8 eggs; 18 eggs were hatched, with the hatching rate of 54.5%; 13 chicks flew, with the flegling rate of 72.2%. The reproductive success rate was 39.4%, and the reproductive productivity was 2.2 ± 1.2 (Table 1). During the breeding period in 2020, there were 10 pairs of Crested Ibis in Yangxian, all of which were successful in breeding, with a nesting success rate of 100%; a total of 24 eggs were laid, with an average clutch size of 2.4 ± 0.8; 18 eggs were hatched, with the hatching rate of 75%. The fledgling rate was 100%, the reproductive success rate was 75%, and the reproductive productivity was 1.8 ± 0.8 (Table 1). Compared with the breeding population in Yangxian, the spawning time of the first egg of the Beidaihe population was 17 days later, which was consistent with the temperature difference between the two places (Fig.1). Through Mann-Whitney U test, we found that the nest egg number of Beidaihe Crested Ibis was significantly higher than that of Yangxian population, which may be caused by conspecific brood parasitism. Surveillance video showed that the clutch number of eggs of the Crested Ibis in the Beidaihe No. 1 nest was 6, more than twice of the average clutch number of the wild population (2 or 3), and we speculated that the conspecific brood parasitism might be occurred. In addition, it was also observed that the No. 8 nest Crested Ibis had both extra-pair copulations and conspecific brood parasitism. The extramarital female laid eggs in the male's nest, making the clutch number as high as 7. After that, three Crested Ibis were found to incubate their eggs in turn, and two females were found to incubate in the same nest. [Conclusion] The conspecific brood parasitism of Beidaihe Crested Ibis may be caused by high density of artificial nest sites and low concealment in the net cage, while the extra-pair copulations may be related to the high population density and reproductive density. Our results of the abnormal reproduction behavior of the Crested Ibis can provide references for the setting of artificial nest baskets in the acclimatizing cages and the selection of wild provenance, and prompt us to pay more attention to the phenotypic plasticity and ecological adaptability of the Crested Ibis under environmental pressure.