The development of large photovoltaic power stations has become a key path for achieving the carbon peak and neutrality targets. The modular structure of the three-phase Cascaded H-Bridge(CHB) grid-connected converter can easily expand its voltage level and power level, and connect to the grid without heavy transformer. In order to better support the operation of the power grid, this digest introduces the control model and design method for grid forming CHB converter based on droop control. This digest first introduces the overall structure of the CHB grid-connected converter system. The mathematical model of the three-phase CHB grid-connected inverter is derived. Based on this mathematical model, the design of the voltage current dual-closed loop and the outer power loop is introduced. The overall control structure is verified by the simulation.