Comprehensively and accurately exploring the impact of social capital on the long-term economic growth is of great practical significance to the enhancement of the status of social capital in the regional economic development. However, traditional endogenous growth model ignores the effect of time allocation on social capital accumulation and residents' utility. This paper introduces social capital into horizontal innovation growth model by using endogenous accumulation of social capital, and comprehensively considers the non-monotonicity impacts of social capital and leisure time on economic growth. Numerical simulations show that social capital plays a promotion role with a diminishing marginal effect in economic growth, but there is an inverse U-shape relationship between leisure time and economic growth, namely the appropriate increase in leisure time is beneficial to the accumulation of social capital. In order to verify the attribute of production factor of social capital, this paper constructs the measurement indexes of social capital at prefecture-level city level based on the network embeddedness feature of structural social capital. The empirical results show as follows: firstly, social capital has a significantly positive impact on economic growth, and its promotion role strengthens as urban economic development within regions increases; secondly, the output efficiency of social capital in the central and western regions is higher than the one in the eastern region, indirectly reflecting the current situation of excessive accumulation of social capital in the eastern region; thirdly, investment level and technological progress are two important ways to affect economic growth by social capital. These conclusions above provide useful reference for clarifying the economic attribute of social capital, stimulating the economic benefits of social capital, promoting the coordinated development of urban clusters and reducing regional differences.