The present disclosure relates to a method for pre-warning deformation of a casing pipe according to a change feature of the b-value of the hydraulic fracturing induced microseismicity. Based on number distribution and seismic magnitudes of hydraulic fracturing induced microseismic monitoring events, a b-value of microseismicity in a fracturing stage is calculated according to a Gutenberg-Richter relation describing frequency-seismic magnitude distribution in seismology, and whether fault activation occurs in a fracturing process is identified according to a change feature of the b-value. According to the method, a cumulative effect of fault activation in the hydraulic fracturing process is considered, and symptoms of fault activation in the fracturing process can be accurately identified, so as to pre-warn deformation of the casing pipe.