Abstract: Nonlinear stress-strain relation in creeping metal was analyzed.The nonlinear fluctuation in creep damage detection with ultrasound was described.A new nonlinear ultrasonic method using pulse wave was proposed,in which the energy ratio of high frequency band to low frequency band of ultrasonic signal was used to characterize the nonlinearity caused by creeping.The influence of attenuation was discussed,and correction to the new nonlinear ultrasonic method was obtained according to frequency-attenuation relation.The base metal and heat-affected zone (HAZ) of P91 steel weldments in different creeping states were detected with this corrected nonlinear ultrasonic method.And the results showed that the nonlinear ultrasonic parameter increased while the creep time increased,and the new method could distinguish different creep states of the same texture.Therefore,this new nonlinear ultrasonic method could be effective for detecting the creep damage degree of P91 steel weldments.