Abstract: According to the 1896 samples of Sthenoteuthis oualaniensis collected by Chinese light falling-net fishery during the same months (February-May) in the northwest Indian Ocean, this study presents the differences of fisheries biological characteristics of Sthenoteuthis oualaniensis between 2019 (Ei-Niño) and 2020 (normal). The result indicated that the mantle length (ML) range and dominant ML group of 2019 were larger than that of 2020, especially for females. The ML range of females in both years were 129—347 and 284—582 mm, and the dominant ML group were 171—220 and 481—530 mm, respectively. The ML range of males were 138—273 and 94—235 mm, and both of the dominant ML group were 171—220 mm. The analysis of covariance (ANCOVA) showed that the relationships between body weight (BW) and ML of Sthenoteuthis oualaniensis in different years was significantly different between the sexes. By fitting and optimizing the equations and comparing the AIC (Akaike’s information criterion), the relationship between ML and BW of males were most suitable to be expressed by exponential functions, except for the males of 2020 when the relationship between ML and BW of males was most suitable to be expressed by a exponential function. The relationships between ML and BW for both the 2019 and 2020 were the best expressed as power functions, except for the relationship between ML and BW for the 2020 males, which were the best expressed by a exponential function. The gonad maturity composition varied between different years, the females were mostly at immature stages (Ⅰ and Ⅱ) in 2019 but mature stages (stages Ⅲ and Ⅳ) in 2020, while the males were mostly at stages Ⅱ and Ⅲ in 2019 but mature stages (Ⅲ and Ⅳ) in 2020. The age composition and hatching characteristics differed between years, the females ranged from 97—263d and 148—267d, and dominant age groups were 171—200d and 231—260d with hatching dates from June to December and May to October, and peaked in August-September and July-August, respectively. The males age ranged from 131—253d and 133—221d, with a dominant age 171—200d for both years, and the hatching dates were from August to December and September to December, and the peak periods were September-November and October-November, respectively. This study shows that the fisheries biology of Sthenoteuthis oualaniensis in the northwest Indian Ocean varies from climate year to climate.