This paper constructs a load regulation model of coal-fired power station based on PID control algorithm. The linear combination of proportional, integral and differential simulates the inertial delay characteristics of the unit, and the grid control instruction is input as a step signal to realize the dynamic regulation of load. The joint operation model of photovoltaic + coalfired power generation is proposed to integrate the multiple systems, and the coal-fired unit is used as a regulating power source to enhance the use of new energy. Measuring the carbon emission reduction effect through environmental benefit assessment indicators. Construct a decision model for the replacement of installed power generation units to provide decision support for the replacement of old units. Demonstrate the environmental benefit optimization ability of this paper’s method through empirical analysis. The results show that greater corporate value can be realized by replacing more coal power installations with photovoltaic installations. The difference between the ideal value and negative ideal value of the city in the three environmental benefit indicators does not exceed 0.05, and the mixed load regulation method has sensitivity. Through the method of this paper regulating the proportion of PV installation increased by 13.92% to achieve the optimization of environmental benefits.