Abstract: Super 304H heat-resistant steel was widely used in superheater and reheater pipes of ultra-supercritical units. The problems of high temperature damage, degradation of mechanical properties and deterioration of Super 304H heat-resistant steel arose increasingly in power plant with the long-term operation of ultra-supercritical units. As a result, study of high temperature aging process and early damage was meaningful to Super 304H steel. The nonlinear ultrasonic signal was sensitive to micro defects and micro cracks at the micro-nano scale, which can fill the blank of traditional nondestructive testing methods in the field of micro-scale detection. In this work, the microstructure and the precipitates of high temperature aging Super 304H steels were studied, and the quantative analysis between aging time and nonlinear ultrasonic coefficient was described and the mechanism was discussed based on the microstructure characteristics of aging steel samples. The results showed that, the number and size of precipitates gradually increased at the aged temperature of 700℃, and successively enriched in the grain boundary and the crystal with the increasement of aged time, The main components of precipitated phase were bulk Cr₂₃C₆ and Nb（C, N） phase. The Nonlinear ultrasonic increased monotonically as the aged time increased, and the slope of the variation curve could be due to degree of of coherency strain formation. The coherency strain state depends on the interaction of grains and precipitated phases during aged process.The results demonstrated the feasibility of non-linear ultrasonic evaluation of thermal damage of Super 304H steel, and provided a theoretical basis for nonlinear ultrasonic detection of Super 304H steel under high temperature conditions.