Concerning the issues regarding driving mileage reduction for electric vehicles (EVs) in cold climates, a heat pump system with low global warming potential refrigerant R290/R1234yf is employed as one of the promising solutions. Different from the widely used mobile refrigerant R134a, R290 and R1234yf are both flammable or explosive. The application of R290/R1234yf in the mobile heat pump system is hindered by unexpected refrigerant leakage with the existence of fire and explosion risk. In this study, the combustion characteristics of R290/R1234yf in a potential leakage process from an air-conditioning heat pump system for EVs were investigated. Firstly, thermodynamic behaviours of R290/R1234yf used in a typical heat pump system were analysed based on a special experimental facility designed for EVs. Then the leakage and combustion characteristics of R290/R1234yf including flame shape, temperature, radiation etc. were obtained by the experimental method under different initial temperature and mass flow rate conditions. It was found that R290/R1234yf leaked is difficult to ignite at low temperatures, while the blow-off phenomenon of the jet flame would occur at high temperature with high leakage mass flow rate. In addition, the results showed that combustion intensity would be enhanced by the leakage mass flow rate between 30 and 60°C. These results could provide guidance for fire detection and rescue system design for new energy vehicles.