[Objectives] Understanding species distributions, and how they change in space and time, is vital in prioritising conservation or management initiatives. However, the basic ecological data of East Asian Finless Porpoise (Neophocaena asiaeorientalis sunameri) in Changdao waters, such as the distribution, population and it’s dynamic, is least known for lacking of long-term studies. [Methods] Five visual surveys of East Asian Finless Porpoise were conducted from October 2019 to October 2021 using line transect sampling method. Our study area and survey lines are shown in Fig. 1. Based on the monitoring data. we calculated the abundance, density, cluster size, encounter rate, spatial and temporal distribution of East Asian Finless Porpoise. The abundance and density analysis were estimated in Distance software and distribution characteristics were conducted in geographic information system (GIS). The remaining statistical analyses were performed utilizing the SPSS statistical analysis software. Kruskal-Wallis test and Mann-Whitney U test were used to examine variations in encounter rate and cluster size across the five surveys. Furthermore, the differences between the number of animal observations by each observer were analyzed using t-test. The statistical significance level was set at P < 0.05, with P < 0.01 indicating highly significant. Mean values for encounter rate, quantity, and density were expressed as mean ± standard deviation (Mean ± SD). [Results] In total, 614 sightings with 1 156 individuals of East Asian Finless Porpoise were recorded over a total transect length of 2 421 km (Table 1). The mean encounter rate of the East Asian Finless Porpoise during the five surveys exhibited its peak in May 2020, reaching 0.49 ± 0.14 groups/km, and reached its lowest point in October 2020, with a value of 0.13 ± 0.02 groups/km (Fig. 2). Similarly, the average group size of observed East Asian Finless Porpoise during the five surveys displayed its highest value in May 2020, recorded as 2.03 ± 0.98 ind/group, while the lowest value was observed in October 2021, measuring 1.60 ± 0.61 ind/group (Fig. 5). The Kruskal-Wallis test showed that there was no significant difference among the encounter rates of the populations surveyed in the same month (P > 0.05), however, significant difference was detected in different months (P < 0.01) (Fig. 3). The results of population density and abundance calculations for the East Asian Finless Porpoise are presented in Table 2. Within the waters of Changdao, the average population density of the East Asian Finless Porpoise in May and October was recorded as 0.56 ± 0.11 ind/km² (0.46﹣0.68 ind/km²), and 3.63 ± 0.25 ind/km² (3.38﹣3.88 ind/km²), respectively. Correspondingly, the average population size of the East Asian Finless Porpoise in May was 20 209 ± 202 ind, exhibiting fluctuations between 20 066 and 20 352 ind. In October, the average population size was 2 971 ± 681 ind, fluctuating between 2 195 and 3 473 ind. The spatial distribution characteristics of East Asian Finless Porpoise was similar that the northwest and southeast sides of Tuoji Island-Daqin Island were the core distribution zone, and it is recommended to focus on these two spots for conservation (Fig. 4). [Conclusion] The population density of East Asian Finless Porpoise in Changdao Waters is the highest among all known areas, and there is obvious seasonal migration. These results represent the first ever estimates for the East Asian Finless Porpoise over a wide portion of the Changdao Waters, with the potential to be useful baseline data to inform conservation.