Based on the perspective of enterprise investment structure, this paper studies the effect of monetary policy on the micro enterprise investment, explains how institutional environment affects the transmission mechanism of monetary policy. It is showed that the tightening monetary policy have obvious inhibition on investment of fixed assets and R&D, but the effect on long-term equity investment is not significant. At the same time, according to different institutional environment, government intervention has a positive correlation to over investment, and both financial development level and the legislative level are negatively correlated to over investment. When local government intervention is high, enterprise more prone to excessive investment behavior. And the high local financial development level and the legislative level can restrain the over investment.