Green credit is an effective financial policy tool to promote green and low-carbon transformation. To analyze whether green credit constraints (GCC) can affect the investment decisions of heavily polluting enterprises (HPEs), this article studies the impact of GCC on green investment decisions of HPEs from 2013 to 2022. Based on the data of bank loan amount, interest rate and maturity of enterprises, this paper uses the entropy weight method to construct the bank loan constraint index of HPEs under green credit, and investigates the nonlinear effect of GCC on enterprises' green investment preference and its main mechanism. The results show that: (1) there is a U-shaped relationship between GCC and green investment ratio of HPEs. (2) Corporate R&D expenditure, financial asset investment, and investment term structure are the main influencing mechanisms. (3) Government environmental regulation and public environmental supervision play a positive and negative regulate role respectively. (4) The results of heterogeneity test show that green credit policy (GCP) is more significant in the eastern region, state-owned enterprises, electric power industry, chemical industry, and material industry. The research conclusions provide targeted suggestions for promoting the development of GCP and accelerating the green transformation of HPEs.