Abstract: Ice crystal icing is an essential menace to flight safety. The inhaled ice crystals by the engine will be partially melt into liquid water due to the high temperature of the internal flow, and the mixed phase conditions will cause ice accumulated on the internal surface of the engine, which may give rise to the engine surge, stall, and even internal structural damage due to ice shedding. Ice crystal icing under mixed phase conditions on the NACA0012 airfoil is numerically investigated. The mechanism of melting rate on ice accretion and the influences of ambient temperature and Mach number on the icing shape, collection efficiency and icing growth rate are analysed. Whether the ice accretion can reach the maximum icing thickness under the mixed phase condition, depends on to what extent the ice crystals and liquid water content can be reached. The ambient temperature directly affects the change of the wet bulb temperature, and as the ambient temperature increases, the thickness and wetting limit of the liquid film also increase. In addition, reducing the ambient temperature or increasing the Mach number dramatically increase the amount and rate of ice formation at the stagnation point of the airfoil leading edge. Current research paves the way for further research on mechanism of ice crystal icing accumulation.