Grid-forming (GFM) converters are widely used because of its great performances of grid-connected. However, the coupling between active power and reactive power makes hard to realize independent control of active power and reactive power. The power coupling has adverse effects on control performance, such as synchronization instability and power oscillations. In order to solve the issue of the power coupling, the paper analyses the nature of power coupling with a virtual synchronous generator (VSG) grid-connected system. A power decoupling control is proposed based on the relationship between active power and reactive power. The voltage compensation and the angle compensation are employed in the power loop to reduce the power cross-coupling. Finally, by comparing with traditional VSG, the simulation results verify the effectiveness of proposed decoupling control in the paper.