As a parameter to characterize the creep state of metal materials, it usually requires the emission ultrasonic probe and the receiving probe to be in coaxial position in penetration detecting. It is difficult to align the position of the emission probe and the receiving probe accurately when the penetration method is used for detecting and acquisition. A set of reflective nonlinear ultrasonic experiment system is designed to solve the problem. The detection information is extracted by analyzing the reflection signal of a bottom wave. The creep properties of P91 specimen with weld structure and creep time of 0 h, 120 h and 250 h were detected by the experiment system. The results show that the ultrasonic second harmonic nonlinear parameters after the 120 h and 250 h creep time of the P91 steel test block, the relative change of the base metal area is 2. 9%, 17. 4%, the weld zone is 2%, 23. 6%, the heat zone is 5. 6%, 34%.