Sardinops melanostictus is mainly distributed in Northwest Pacific Ocean, which is significantly affected by climate change. The carbon and nitrogen stable isotope ratios of the muscles of Japanese sardines from the Northwest Pacific Ocean were measured based on samples taken by the ocean-going fishery resources survey ship “Songhang” in the high seas of the Northwest Pacific Ocean between June and July 2021. The GAM model was then used to examine the effects of both biotic （body length） and abiotic（Latitude, sea surface temperature, offshore distance, chlorophyll-a） factors on the stable nitrogen isotopes； The differences in the trophic niche among sex and body length groups were also analyzed in Japanese sardines in Northwest Pacific Ocean. The results showed that there was no significant difference in muscle δ¹³C and δ¹⁵N between males and females during La Niña years. However, both δ¹³C and δ¹⁵N differed among body length groups. The GAM model's results showed that as body length increases, both δ¹³C and δ¹⁵N exhibited a trend of first decreasing and then increasing； δ¹³C also exhibits this trend with increasing latitude； δ¹⁵N exhibits a negative correlation with both offshore distance and chlorophyll-a. The overlap rate between female and male niche is generally high, with a similar niche width between females and males ranging from 100 to 160 mm. The width of the ecological niche varied across the length groups, and its position changed with growth. However, compared to males, females have substantially broader niches, ranging from 161 to 220 mm. In contrast to usual years, the La Niña year's readings for δ¹³C and δ¹⁵N are all low. The study demonstrates that the feeding ecology of Japanese sardines in Northwest Pacific Ocean varies depending on sex and body length, and that changes in the marine environmental elements under La Niña events affected the feeding ecology of Japanese sardines.