Abstract: The cyclic soft stimulation (CSS) is a new method of reservoir reforming for which the mechanism of fracturing crack propagation is ambiguous with regard to the alternating fluid pressure. This study aims to provide a comprehensive understanding of the fracturing mechanical characterizations of CSS under different magnitudes and amplitudes of the alternating fluid pressure. Acoustic emission (AE) is recorded to investigate the damage evolution under CSS based on the b value analysis of AE. Experimental results reveal the difference of pressure in a crack under different cyclic fluid pressure conditions. The AE results show that the maximum radiated energy under CSS tends to be reduced with the increase in the amplitude and magnitude of the alternating fluid pressure. The finishing crucial touch is that the crack extending criterion under CSS is proposed, which combines the injection parameters, the rock properties and in-situ stress. According to the crack extending criterion, the fluctuation fluid pressure causes the reduction of a critical crack extending pressure, and the CSS causes the crack to initiate and propagate under low fluid pressure. Under a higher-value magnitude of alternating fluid pressure, the cyclic times of CSS is less for the crack initiation. In supplement to the crack extending criterion, a distinct relationship between the radiated energy and the cyclic fluid pressure also is established based on the energy dissipation criterion. These new findings provide an insight into the determination of crack extending criterion under CSS for efficiently implementing shale fracturing.