The invention provides a well drilling method for using dry ice drilling fluid to assist in rock breaking and belongs to the technical field of hot dry rock geothermal well drilling construction. Granular dry ice serves as drilling fluid, heat absorption during different phase state changes of carbon dioxide is sufficiently utilized to cool a drilling tool, temperature gradient is generated on rock mass in front of a drill bit, the rock mass is subjected to thermal fracture, then the properties of the rock mass are weakened, and the rock breaking assisting function is achieved. In the circulation process of the dry ice drilling fluid, the functions including (1) drill bit cooling and lubricating, (2) rock breaking assisting, (3) rock waste carrying and (4) well wall system stability maintaining are brought into full play. Meanwhile, cyclic utilization of the dry ice drilling fluid is achieved on the ground. Through the well drilling method, the difficult problem of well drilling under high temperature and high pressure conditions can be solved, the technical problems that due to the high-temperature and high-pressure environment, the drilling tool stability is poor, the performance of the drilling fluid is poor, the drillability of a rock stratum is poor, the stability of a drill hole is poor, and the well drilling construction cost is high are solved, and the technical defects of hot dry rock geothermal development well drilling construction are effectively overcome.