The invention relates to a traction conversion device of a motor train unit, in particular to a main circuit of a traction converter for a high-speed motor train unit. The main circuit of the traction converter of the high-speed motor train unit comprises a first input voltage/current buffer circuit, a second input voltage/current buffer circuit, a first four-quadrant rectifier, a second four-quadrant rectifier, a secondary filter circuit, a voltage sensor, a support capacitor, a first chopping circuit, a second chopping circuit, a first inverter, a second inverter, a first contactor circuit, a second contactor circuit, a first capacitance circuit and a second capacitance circuit. The main circuit of the traction converter of the high-speed motor train unit, proposed by the invention, has the advantages of energy saving, low heat generation, high protection capability, high reliability and the like, is applicable to various mode running, and the problems of high heat generation, high thermal loss and high cost of the converter of the existing permanent-magnet synchronous motor are solved.