Abstract: Gas detonation is close related to mixability of fuel/air.In order to investigate on the mixing of fuel and air in the detonation tube after filling,numerical simulation of filling and mixing processes of fuel/air in the detonation tube was performed by a CFD software,and the influence of inlet(diameter,) mass ratio,inlet velocity on the distance needed for the uniform mixing of fuel/air was researched.The result showed that,the distance needed for the uniform mixing of fuel/air has a relationship with the ratio of fuel inlet velocity and air inlet velocity.The bigger the ratio was,the larger the distance needed for the uniform mixing of fuel/air was.