The piping system with special safety requirements or higher requirements for structural completeness in nuclear power field, are not allowed to install the temperature sensor through opening pore in the pipe to measure the pipe wall temperature and fluid temperature. A kind of indirect and nondestructive method is in need to predict or evaluate the distribution and variations of the inner wall temperature. The mathematical model of temperature field is established based on the finite element method. And the inverse problem analysis is based on the conjugate gradient methods. The third boundary condition－the inner wall temperature, the fluid temperature near the inner wall, and the convective heat transfer coefficient－is simultaneously inverse solved, and the whole picture and distribution of the wall temperature are obtained. The calculation results and the error analysis indicates that the method is with high accuracy and can inverse estimate the third boundary condition precisely. The method can not only offer exact transient thermal load for the structure stress analysis and thermal fatigue analysis, but also provide a solid theoretical basis for achieving a kind of nondestructive measurement method.