Abstract: In this paper the weldability of 1Cr18Ni9Ti and 15CrMo steels is analysed by magnetic recongnizer and scanning electromicroscope.Main factor of forming martensitic shortness layer is discovered in the weld at normal temperature and continuous high temperature.The effect law of different welding material and technology on ingredient and microstructure and performance of joint are discussed.And change law of martenstic shortness layer conent at different state is also discussed.A effective method of pre surfaced 8 mm thick transition layer of pearlitic new R417 electrode,then depositing austenitic A307 complex alloy on 15CrMo side beveled surface in 1Cr18Ni9Ti and 15CrMo joint has been created.This method has fully eliminated martensitic shortness layer of dissimilar steel joint at the normal temperature and high temperature.It solved welding problem of pipeline joint in ammonia synthetic tower and provided some reference for joint in other chemical pressure equipment.