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Plenty of soft inclusions such as micro pores and micro cracks are commonly distributed in concrete. Figuring out the generation and propagation of micro cracks in concrete is of great significance to the detection and evaluation of initial damage in concrete. Second harmonic generation method is more sensitive to the micro damage in concrete. However, the study of the regularity of nonlinear ultrasonic during crack propagation is rare and insufficient. Therefore, the integrity of concrete specimens was first verified by observing the microscopic morphology of the samples using scanning electron microscopy (SEM). Then a simultaneous test system for stress, strain and nonlinear parameters was designed to investigate the regularity of the nonlinear parameter β during the compressive loading. The results show that the nonlinear parameter β increases significantly at the beginning of loading. With the increase of the compressive load, the nonlinear parameter β decreases rapidly, and fluctuates during the subsequent loading. Compared with the first wave amplitude A, it is proved that the nonlinear ultrasonic parameter β is more sensitive to the initiation and accumulation of the micro damage in concrete, and the first wave amplitude A has a better regularity to the later elastic stage and the plastic stage in concrete. [ABSTRACT FROM AUTHOR]