A non-steady model of propellant droplet evaporation in inert gas has been developed.The model equations have been solved by numerical method with full implicit central scheme.The results show that

when the droplet diameter keeps unchanging

there exists a special pressure frequency

to which the droplet evaporation responds most strongly.In this case the droplet evaporation oscillation approaches to its maximum as the droplet temperature comes near the equilibrium temperature.Therefore

it is reasonable to believe that propellant evaporation could trigger unstable combustion in the rocket engine.