[Objectives] Breeding period occupies an important stage in the life history of cephalopods, we study their reproductive needs by observing their behaviors. [Methods] In June 2020, we observed directly on 30 Octopus sinensis reared in an indoor pool during their breeding period as well as by underwater photography. [Results] The behaviors such as movement, predation (Fig. 1), courtship, mating (Fig. 2), oviposition and brooding (Fig. 3) were found during the spawning period. The power to conduct swimming and crawl in O. sinensis was produced by blowing water with their funnel to move short distanceforward, and they retreated quickly when they were shocked and occasionally as well as jetting ink. Before spawning, O. sinensis actively hunted and consumed Charybdis japonica. After spawning, the appetite of females decrease and finally stopped eating and males lost their appetite gradually. Courtship and mating occurred frequently during spawning period. Males pursued females actively and tended to mate with females with the similar body size, and they also to fight against each other for mating. O. sinensis was polygamy, and kept a certain distance when mating. Males and females turned their body dark brown during mating, and males extend their arms to keep other males away from their mating female. Males could mate with different females, and vice versa. There was a phenomenon of one female mating with multiple males at the same time. Males matured earlier than females, rather than laying eggs immediately after mating, females usually waited for maturation of gonads. Some individuals begun spawning even after more than 50 d. O. sinensis tended to lay their eggs in their nests and sealed them by stones. If there was no nest, the fertilized eggs were protected by covering them with their bodies. Females cleaned the dead eggs and blew the fertilized eggs to provide oxygen after spawning. The fertilized eggs were translucent yellow and elastic, the average diameter was (2.5 ± 0.2) mm × (1.1 ± 0.1) mm. The average fecundity was 83 988 eggs (Table 1). Females had the brooding behavior, the incubation time was between 21 and 24 days under 22.4﹣23.5 ℃. After hatching, the body weight of female was 46% lower than that before oviposition, and died gradually. The longest survival time of brooding female was 34 d, and the longest survival time of mating male was 56 d. [Conclusion] We conclude that the polygamy of O. sinensis could improve their mating success rate, and the brooding behavior could benefit to larval hatching.