It is hard for the current power grid project investment model to simultaneously involve Transmission and Distribution Price (TDP) and construction timing. A novel multi-stage investment decision optimization method for power grid projects under the background of TDP reform is proposed. First, we use the electricity price accounting method after TDP reform to calculate the cost and income of grid project investment. Then the investment period is divided into several stages. Equipment investment is made at the beginning of each stage to meet increasing grid load demand. Taking the highest return in the entire investment period as the objective function, using power balance, maximum projects number and precursor relationship etc. at each stage as constraints, a multi-stage investment optimization decision model for power grid projects investment under TDP Reform is established. Finally, an example based on the IEEE 24-bus test system is used to verify the effectiveness of the model and demonstrate its superiority in improving investment efficiency. This work is supported by the National Natural Science Foundation of China (No. 51907123) and Economic and Technological Research Institute of State Grid Jiangsu Electric Power Co., Ltd. (No. SGJSJY00JJJ2000057).