The stress rupture strength is of great significance to evaluate the service life of the C-HRA-5 welded joint under complex working conditions. A mathematical model was established to predict the stress rupture strength based on the LarsonMiller（L-M） parameter formula through the high-temperature short-time tensile test at different temperatures, and the stress rupture strength of 100 000 hours was extrapolated. The results show that the welded joint of C-HRA-5 steel represents the best strength and plasticity at 650 ℃ at all service temperatures, and presents typical micropore aggregation fracture at 700 ℃. With the increase of temperature, the second phase precipitates in-crease, the dislocation pile-up increases, and the mechanical properties of the welded joint decrease. Based on L-M parameters and the characteristics of state function and total differential, a mathematical model is established to predict the stress rupture strength of C-HRA-5 steel welded joints. The extrapolated fitting formula is lgσ =2.760 18-0.131 51 lgτ at 700 ℃ and the lasting strength of 100 000 hours is σ,10,5,973,=116.020 3 MPa, higher than ASME SA-213 standard.