In this paper, six groups of welding heat input were used to weld the 25Cr35NiNb+microalloyed ethylene cracking furnace tubes. The weld structure and high temperature durability of each group of welding heat input were studied.The results show that under the condition of 1 050 ℃ and 25 MPa,when the welding heat inputs are 9 kJ/cm and 11 kJ/cm,the high temperature lasting fracture time are 132 h and 119 h respectively. When the welding heat inputs are 6 kJ/cm,8 kJ/cm,12 kJ/cm and 16 kJ/cm, the high temperature lasting fracture time are 78 h,84 h,105 h and 109 h respectively. When the range of welding heat inputs is 9～11 kJ/cm,the 25Cr35NiNb+microalloyed furnace tube welding joint can obtain longer high temperature rupture time. The microstructure observation shows that the microstructure of weld metal is obviously different under different welding heat input,which is the main reason for the change of high temperature durability of welded joint.