The invention discloses a multi-terminal flexible direct current transmission system and a method for confirming the voltages of voltage sources. The system comprises a direct current transmission line circuit and a convertor station calculation circuit, wherein the direct current transmission line circuit is used for transmitting electric power. All input ports of the direct current transmission line circuit and all output ports of the direct current transmission line circuit are connected with the positive electrode of the convertor station calculation circuit and the negative electrode of the convertor station calculation circuit respectively. The negative electrode of the convertor station calculation circuit is grounded. The convertor station calculation circuit comprises three sets of RLV branch circuits which are connected in parallel. Each RLV branch circuit comprises the corresponding voltage source, an inductor and a resistor, wherein the voltage source, the inductor and the resistor are sequentially connected in series. By means of the multi-terminal flexible direct current transmission system, the maximum overvoltage on a direct current transmission line can be confirmed. Compared with a calculation circuit of an existing convertor station in the prior art, the convertor station calculation circuit is simple in structure and saves resources.