The invention provides a prebias switch machine circuit and a prebias circuit and method of a multi-cascade converter, and is applied to the field of communication. The prebias switch machine circuit comprises a detection module and a control module, wherein the detection module is connected with an output end of the multi-cascade converter and is used for detecting an output signal of the multi-cascade converter, and the control module is connected with a push-pull circuit of the multi-cascade converter and is used for generating a prebias signal according to the detected output signal when the output signal is a preset output signal and controlling a rectification tube driving switch circuit of the push-pull circuit according to the prebias signal. Compared with the prior art, the prebias switch machine circuit has the advantages that the prebias switch machine circuit is additionally arranged in a DC converter, whether a prebias switch machine is needed or not is judged by detecting the output signal of the multi-cascade converter, the prebias switch machine circuit is simple to implement, and accurate control of an entry point is achieved. By the prebias switch machine circuit, the problem of device damage due to excessively large reverse current of a freewheeling switching tube caused by a prebias voltage of a load is solved, and the drop of an output voltage of the converter is also prevented.