The invention discloses a self-adaptive switching method, device and system suitable for a power electronic transformer. The method comprises the steps: acquiring the output active power of a power electronic transformer in an initial state, wherein the power electronic transformer comprises a plurality of sub-power modules, and each sub-power module comprises a preceding-stage AC/DC converter anda backward-stage high-frequency isolation module; based on the obtained output active power of the power electronic transformer, adaptively adjusting and controlling the number of input operation modules of a high-frequency isolation module in the power electronic transformer according to a set strategy, obtaining a power electronic transformer in a new state, and adjusting and controlling the d-axis component and the q-axis component of the alternating-current output voltage of each sub-power module in the power electronic transformer in the new state, thereby achieving the optimal distribution and stable operation of the power of the modular power electronic transformer. According to the invention, only a part of the high-frequency isolation modules are kept to supply power in parallelunder the light-load working condition, and the overall efficiency of equipment is effectively improved.