Abstract: The mechanisms on ice crystals generation triggered by non-catalysis method, including blast by the compressed air (burst of the film and shock tube) and detonation of powder, are investigated. The shock wave-high pressure mechanism and the supersonic cavitation mechanism are preliminarily negated at first based on qualitative experimental while the supersonic adiabatic expansion is considered to be the true mechanism leading to ice crystals generation. Then, a supersonic air flow generator for triggering ice crystals generation in supercooled fog is designed. The experimental results show that a supersonic air flow (M=1.1) has a great effect on produce of the ice crystals. The threshold temperature is -20C. The critical pressure is 2.1 atm. The threshold pressure increase as the experimental temperature increase. A large number of ice crystals are produced when the total pressure is 0.2-0.3 atm higher than the threshold pressure. The results are consistent with the theory on supersonic expansion.