The invention discloses a frozen blocking preventing co-production LNG liquid nitrogen wash device combined with a mixed refrigerant system and a method thereof. The frozen blocking preventing co-production LNG liquid nitrogen wash device is mainly composed of a molecular sieve adsorption system, a cold box separation liquefying system and a mixed refrigerant refrigeration circulation system. The molecular sieve adsorption system is mainly composed of adsorbers and a regeneration system, and carbon dioxide and methanol in raw material gas are removed through a mature liquid nitrogen wash molecular sieve adsorption technology. The mixed refrigerant refrigeration circulation system is mainly composed of a compressor unit and a refrigerant storage and distribution unit; a compressor adopts a singular MRC centrifugal compressor; and energy consumption is low, the equipment is simple, and stability and reliability are achieved. The cold box separation liquefying system is mainly composed of a plate-fin heat exchanger set, a nitrogen wash tower and a methane rectifying tower. By means of the frozen blocking preventing co-production LNG liquid nitrogen wash device combined with the mixed refrigerant system and the method thereof, purifying gas coming from the upstream low-temperature methanol washing process is received, CO, CH4, Ar and other impurities in the purifying gas are removed, pure synthesis gas synthesis ammonia with H2/N2=3:1 is prepared, meanwhile, and after separated methane-rich liquid is separated through the rectifying tower, LNG is obtained and fed to an LNG tank field outside battery limit.