The invention relates to a method and a system for optimizing the power-on mode of a multi-fed DC transmission system. The method comprises the steps of determining a DC fault of a DC incoming stationand a generator set, wherein the DC fault is maximum in reactive variable quantity and the reactive output proportion of the generator set is larger than a preset threshold value under the conditionof the above fault; adjusting the power-on mode based on the generator set with the reactive output proportion thereof larger than the preset threshold value under the condition of the above fault; based on the power-on mode, carrying out the safe stability calculation, and determining the power-on mode of the multi-fed DC transmission system. According to the method and the system, the power-on mode is adjusted through screening out the generator set with the reactive output proportion thereof larger than the preset threshold value. Therefore, the voltage of an AC-DC system containing the multi-fed DC transmission system is kept stable.