Soft coal seams with coal and gas outburst tendency do not have impulsion tendency, but rock burst disasters occur in soft coal seams under mining in the deep stope, and the existence of this dynamic disaster is confirmed by literature research and cases analysis. The unconfined unidirectional compression test and bursting liability identification indicated that coal samples were not prone to burst, and the monotonic strain strength softened after ultimate load. To study the mechanism of bursting disasters in soft coal seams, the boundary constraints and loading conditions of the excavating face and the mining face under the engineering background were simulated, and the pressure test under the single-degree-of-freedom border was carried out to investigate the stress, strain, and acoustic characteristics of the whole disaster process. The results showed that when coal samples loaded under the boundary condition of single-degree-of-freedom, the strain strength of all the three coal samples appeared as the process of softening-hardening-bursting, and the softening-hardening process even arose repeatedly. The strain strength hardening degree before bursting was remarkable, and the hardening coefficients of the three coal samples were 1.6,1.53, and 2.25, respectively. After strain strength hardening, the compression strength of coal samples before bursting disaster exceeded the threshold value of bursting tendency. Under the pressure with the constraint of single-degree-of-freedom, the bursting disaster, similar to that happens in hard coal seams, occurred when the strain hardening met the strength requirement of bursting, which reasonably explains the cause of rock burst in soft coal seams. When excavating and mining in soft coal seams with coal and gas outburst tendency under the condition of high stress in the deep mines, it is necessary to pay attention to rock burst caused by strain strength hardening. The intensity of such impulsion is generally not high in engineering, but it can induce highly harmful coal and gas outburst or unusual gas emission.