Abstract: Public S&T subsidies are important means to solve the externality of corporate R&D activities and promote the innovation-driven development. Based on the data of large and medium-sized industrial enterprises in China during 2009 to 2014, this paper explores the effects of three kinds of ways, including direct subsidies, direct tax credits and indirect tax credits, of public S&T subsidies on private R&D expenditure, and also analysis the moderating effects of other factors like intellectual property right protection, enterprise scale, property types, knowledge stocks and R&D capital stocks. The research found that all three kinds of public S&T subsidies have significant positive effects on private R&D expenditure, especially for direct tax credits, which has the strongest. However, the moderating effect of intellectual property right protection is not significant. The positive effects of public S&T subsidies on small scale enterprises and private enterprises are stronger than large scale enterprises and state-owned enterprises. The direct tax credits has a stronger positive effect on those corporate who has large R&D stocks and knowledge stocks, and the direct subsidies and indirect tax credits have stronger incentive effect on those has small one.