To study the physical mechanism of the premature-combustion phenomenon in the FAE explosion process, propylene oxide was used as the FAE explosion fuel. Based on the theory of fluid mechanics, numerical simulation software ANSYS was used to simulate the fuel dispersion process of a 2 kg FAE device. The results show that the central charge has a significant impact on the premature-combustion behaviour. After the detonation of central charge, high temperature and high pressure are generated to promote the fuel dispersion. Due to the mechanism of heat conduction and thermal radiation, the temperature field and the pressure field also exist in the FAE cloud field formed by the fuel dispersion. Premature-combustion occurs when the temperature and concentration density of the FAE cloud field meet critical conditions. Numerical simulation indicates that the location of the premature-combustion approxiamtely locates near the top of the FAE device, and the premature-combustion time is concentrated within 1~5 ms after the fuel begins to disperse.