An embodiment of the invention discloses a convertor station off-stream control method and device thereof for a multi-terminal flexible direct current transmission system. The multi-terminal flexible direct current transmission system comprises at least three convertor stations; the convertor stations are modularized multilevel converters; the DC (Direct Current) side of every single converter station comprises a DC current detector and at least a group of isolation switch blades and the AC (Alternating Current) side comprises a circuit inlet circuit breaker, a convertor transformer and a charging resistor which are in series connection mutually. The convertor station off-stream control method comprises switching converter valves in the converter transformers at converter stations which are required to be in off-stream to be in high-resistance states; detecting the DC side current of the converter stations which are required to be in off-stream; judging whether the DC side currents at the converter station stations which are required to be in off-stream are smaller than a preset value or not; disconnecting the isolation switch blades on the DC sides of the converter stations which are required to be in off-stream if the DC side currents at the converter stations which are required to be in off-stream are smaller than the preset value. The multi-terminal flexible direct current transmission system achieves the purpose that the operating converter stations are not influenced when partial converter stations in the multi-terminal flexible direct current transmission system is off-stream.