The invention relates to a control mode conversion method for a high-voltage direct current tripolar power transmission system. The method is applied when the high-voltage direct current tripolar power transmission system needs to reduce power for operation or one direct current line breaks down, and particularly includes the following steps that (1), the direct current line of which operation needs to be stopped is determined; (2), convertor stations connected with the two ends of the direct current line determined in the step (1) are shut down, and a direct current switch on the line is switched off; (3), the convertor stations which are shut down send an operating mode changing instruction signal to other convertor stations; (4), other convertor stations change reference values of voltage and current according to the received instruction signal, and the tripolar direct current power transmission of the system is converted to be traditional bipolar direct current power transmission. By means of the method, control modes can be rapidly and stably converted when the high-voltage direct current tripolar power transmission system operates stably or breaks down, and therefore the operation quit rate of individual convertor stations and the stability of the system are improved.