Abstract: In order to study the change characteristics of unit liquid injection volume during hydraulic fracturing of coal seams, using the engineering test method, taking the 1703 working face of Jinzhong Coal Mine in Junlian County, Yibin City, Sichuan Province as the test site. Through monitoring and analyzing the unit liquid flow of each pipeline in the process of borehole hydraulic fracturing, it is found that the fitting curves of unit liquid flow of liquid inlet pipe and liquid return pipe are “parabola” in the process of normal hydraulic fracturing, while the actual unit liquid injection volume can be divided into falling stage and fluctuating stage. Combined with the change of liquid injection pressure in the process of hydraulic fracturing, it is found that the liquid injection pressure will appear obvious “pressure relief-recovery” phenomenon when the coal body appears cracking and crack propagation, and the unit liquid injection volume will appear “ rise-recover ” phenomenon, and the two phenomenon are synchronous. In addition, the whole hydraulic fracturing process is divided into three stages according to the change characteristics of the unit liquid injection volume in the hydraulic fracturing process: filling stage, energy storage and coal cracking cycle stage and stop cracking stage.