Tax incentives are closely related to employees' income. The relationship between tax incentives and firm social insurance contributions is underexplored in existing literature. We construct a theoretical model to portray the impact of tax incentives on firm social security contributions and use China's accelerated depreciation policy for fixed assets (AD reform) to test it empirically. We find that the tax incentives effectively increase the social security contributions of firms. This effect is more pronounced in large firms, firms with high capital intensity, firms with strong bargaining power of employees, and firms with social security contributions levied by the social security department. Moreover, the AD reform promotes improvements in firm productivity and performance by increasing investment in machinery and equipment, increasing the rents shared by firms with employees, and thus indirectly boosting firms' social security contributions. Overall, Our findings contribute to a deeper understanding of rent-sharing between firms and employees, as well as enhancing our understanding of the effective incidence of taxes in less developed regions. •We investigated how tax incentives affect firm social insurance contributions.•China's accelerated depreciation policy for fixed assets allows us to run a difference-in-differences regression.•Tax incentives have a significant and positive effect on firm social insurance contributions.•The indirect improvement effect of rent sharing between firms and employees is the channel through which tax incentives affect firm social insurance contributions.•The effect varies across firm characteristics.