Grid-forming voltage source converters (VSCs) is the key solution for problems like low inertia and weak damping in power system. When operation condition changes, parallelled grid-forming VSCs are facing power frequency oscillation due to their different parameters. This paper proposes power-frequency oscillation control based on electromagnetic power and decentralized mutual damping compensation strategy for Grid-forming VSCs. First, the grid-forming VSCs model is established to analysis oscillation mechanism of grid-forming VSCs by combining the characteristic root trajectory with parameter changes. Then, the electromagnetic power and decentralized mutual damping compensation control are applied to dynamically adjust the mutual damping characteristics of the system and reduce virtual torque and angular frequency error. The root locus method is used to verify the effectiveness of the control strategy. Finally, the effectiveness of the proposed control strategy is verified by simulations under working conditions of different scenarios.