The weldability of P91 and P22 steel are poor,and the dissimilar steel welding joints which are made by the two kinds of steel are very easy to harden and crack. In addition,due to the differences of the two kinds of steel in material properties,welding heat treatment requirements,and the decrease of heat treatment quality caused by the smaller product diameter which is easy influence on heater installation effect,it is difficult to ensure that the dissimilar steel welding joints achieve the comparatively ideal comprehensive mechanical properties,especially hardness index. Therefore,through analyzing material properties and the weldability of P91 and P22 small diameter pipe dissimilar steel in main steam system and according to the welding preparation,welding method,welding material,welding parameters and post-weld heat treatment steps to implement technology test. The results show that joint weld quality inspection,mechanical property test,microscopic metallographic examination,hardness test meet the design requirements and ASME specifications. Through the above implementation contents,the welding technology for on-site welding is finally determined,and the welding technology achieve good result after being applied in actual production,meet the welding construction requirements of P91 and P22 small diameter pipe dissimilar steel,and provide technical guarantee for on-site welding.