Innovation policies, especially government R&D subsidies as a direct innovation policy instrument, play an important guiding role in technological innovation and contribute to the development of new quality productive forces. Government effectiveness is recognized as a key contingency factor influencing the implementation efficiency of such policies. A comprehensive index of new quality productive forces was constructed, and provincial panel data were employed to empirically examine the impact of government R&D subsidies on new quality productive forces. Following a rigorous theoretical framework, the moderating mechanism and threshold effect of government effectiveness were further tested. The results show that government R&D subsidies significantly promote the development of new quality productive forces, and this conclusion remains robust across multiple robustness checks. The subsidy effect exhibits a smooth nonlinear characteristic, where the marginal promotion effect strengthens with increasing subsidy intensity. Government effectiveness exerts a positive moderating effect on the relationship between R&D subsidies and new quality productive forces. Results from the panel threshold model indicate that government effectiveness has a threshold characteristic, and in the interval of higher government effectiveness, R&D subsidies have a stronger facilitating effect on new quality productive forces. Heterogeneity analysis reveals that the policy effect is statistically significant and more pronounced in central and western China, whereas it is not significant in eastern regions under the current sample and model specification. Based on these findings, policy implications are proposed, including improving the design of the innovation policy system, enhancing government effectiveness, and implementing regionally differentiated strategies. It provides empirical evidence for understanding the governance contingency of innovation policy performance and offers insights for optimizing the institutional environment for the development of new quality productive forces.