Abstract: In order to study the gestation period, pregnancy health and fetal development of female California sea lions（Zalophus californianus）, a 10-years-old female California sea lion and a 9-years-old male California sea lion introduced to Dalian Oceanarium in September 2011 were observed with B ultrasound for a period from the time when they were caged together to the birth of pup, and the female sea lion was supplemented with ferrous succinate, garlic oil and lactobacillin tablets during estrus and pregnancy period. The results showed that the total gestation of the cow lasted for about 11 months. Copulation was found for the first time on the 3th day after caging, the presence of gestational sac and yolk sac were first detected in the uterine cavity of the female sea lion on the 37th day after copulation, and the fetal heart and embryo buds were detected on the 44th day. At the 4th month of pregnancy, the girth of the thoracic cavity of the female sea lion was significantly enlarged. At the 5th month, the fetus was formed and the female sea lion showed symptoms such as loss of appetite and poor mental health. At the 6th month, the physical condition of the female sea lion returned to normal. The female sea lion successfully gave birth to a pup in the 11th month of pregnancy.