Aiming at the characteristics of grid oscillation under periodic load disturbance, a method for obtaining system oscillation modal information by mapping spring network is proposed. Firstly, by analyzing the mathematical models of single machine and multi machine systems and single degree of freedom and multi degree of freedom elastic systems, the mapping relationship between grid and spring network model is determined, and then the mechanism of forced power oscillation caused by periodic load is deduced. When the load is periodically disturbed and the disturbance frequency is close to or equal to the natural oscillation frequency of the system, it can induce the system to generate forced power oscillation. Finally, the example analysis further verifies the consistency of the oscillation modes between the grid and the spring network. This method provides a new idea for studying low frequency oscillation and has certain reference value.