Japanese scad (Decapterus maruadsi), a warm water pelagic fish, is an important economic species in China. Based on a quarterly bottom trawl survey in offshore waters of southern Zhejiang from 2015 to 2018, this study explored the length-weight relationships, growth heterogeneity, and natural mortality of Japanese scad. A total of 1,134 individuals were measured, with fork length (L) ranging from 45.0 to 247.0 mm and body weight (W) ranging from 0.7 to 206.6 g. The length-weight relationship was revealed to be W=5.01×10^-6L^3.17 for Japanese scad in the inshore waters of southern Zhejiang. A linear mixed effect model considering the random effects from season, gender, and year performed the best, with the lowest AIC and RMSE values. Results from the best model indicated that 1) individuals collected in the autumn tended to gain more weight than those collected in spring and summer at the same length; 2) individuals collected in 2018 grew with better condition, whereas those in 2017 and 2018 gained less weight at the same length; 3) there was no substantial differences between female and male Japanese scad. Therefore, year and season significantly influenced Japanese scad growth in the inshore waters of southern Zhejiang with the best growth condition in autumn 2018. Based on several empirical equations including Pauly, Pauly update, Jensen, Hoenig and Lorenzen methods, this study estimated the natural mortality for Japanese scad, which ranged from 0.36 to 1.41. This study exhibited that season and year significantly influenced the length-weight relationships of Japanese scad and the results could provide important information and knowledge for further study and fishery management for this important species in the coast waters of China.