The invention belongs to a micro-flow gas separation device, and particularly relates to the deuterium and helium mixed gas separation device in the technical field of vacuum leakage detection of nuclear fusion devices. The device includes a housing; the housing is of a T-shaped channel structure with double openings; the vertical direction of a T shape in the housing is packaged with a refrigerator, and the horizontal direction of the T shape in the housing is internally provided with a condensing plate; a baffle plate is arranged on an end port, and the upper end is provided with an upper radiation screen; the baffle plate is connected with a first-level cold head of the refrigerator; the condensing plate is connected with a second-level cold head; a lower radiation screen is arranged on the refrigerator in a sleeved mode. The micro-flow gas separation device has the advantages that the low-temperature condensing plate condenses deuterium gas in micro-flow residual gas, but does not generate an adsorption effect on helium gas, so as to reach the purpose of separation of the deuterium and helium mixed gas.