With good thermal performance and environmental characteristics, R32 is an alternative refrigerant of R22, but its application is limited by its flammability. In view of indoor security, this paper analyzed the leakage and diffusion of R32 of wall-mounted air conditioners under different operating conditions. The impact of air supply velocities and the leakage hole positions on the concentration distribution was also analyzed when the refrigerant leaks from the indoor unit while the air conditioner is operating. The results show that the leakage rate of R32 decreases with air conditioner operation time. The leakage process of refrigerant under the operating conditions can be classified into a fast leak stage and a slow leak stage. When the refrigerant leaks from the inlet pipe of the evaporator, the leakage rate is higher than that from the outlet pipe of the evaporator. This paper also analyzed the impact of air supply velocities on the concentration distribution and obtained the concentration distribution under different operating conditions. The concentration distribution was compared when the R32 leaks from different positions. Then, the indoor security was evaluated when R32 leaks from an operating air conditioner. And the results suggest that the combustible zone only appears near the leakage hole and the duration of the combustible zone is very short. We can infer that the fire risk of R32 as the alternative refrigerant of an air conditioner is very low under the general operation conditions. (C) 2016 Elsevier Ltd. All rights reserved.