Onshore wind power full DC power generation system can effectively solve the problems of harmonic resonance and reactive power transmission in the process of grid-connected large-scale wind power. But the transient stability mechanism under large disturbances is different from the traditional AC system due to the many types of power electronic devices and the complex control coupling of each device in the system. In order to investigate the large disturbance stability mechanism of onshore wind power full DC power generation system, firstly, the system equivalent model is established based on the operation characteristics and control strategy of key equipment in the full DC system for the scheme of "DC convergence-AC output". Then, based on the Brayton-Moser mixed potential function theory, the large disturbance stability mechanism of the onshore wind power full DC power generation system is analyzed. By solving stability criterion, the influence of key parameters on the system large disturbance stability is analyzed. The results show that the stability of the system is related to both the system parameters and control parameters of each converter. Finally, a simulation model is built based on MATLAB/Simulink to verify the correctness of the theoretical analysis.