Abstract: Usually, the liquid sodium droplet can ignite spontaneously when it appears in normal temperature air,which is different from the ignition characteristics of hydrocarbon fuel droplets. The ignition delay time and ignition limit of sodium droplets are controlled by the pre-ignition stage before gas phase combustion. In the pre-ignition stage, surface oxidation is the main process. The characteristics and physical model of this stage is a key point in the study of sodium droplet combustion. At present, the pre-ignition model used in the safety analysis program of the spray sodium fire of fast reactor in China controls the surface oxidation reaction rate by the rate of oxygen transfer to the surface of sodium drops, and the ignition delay time is calculated from the temperature of the sodium drop to the boiling point（1 156 k）. The temperature based chemical kinetic control model and the combined mass transfer/reaction kinetic control model have been developed to describe the pre-ignition stage of sodium droplet. Through the comparison of the calculation and analysis results of different models for different initial conditions of sodium droplet ignition, the latter two models can predict the ignition limit of sodium droplet under different conditions and more in line with the actual ignition characteristics of sodium droplets.