The invention discloses an injection slurry diffusion range and rule determining method during anchor rod slurry injection. The determining method comprises finishing installation of an anchor rod during underground chamber engineering anchoring and slurry injection, and slurry injection, primarily determining a diffusing range of the slurry in surrounding rocks via geological observation, building a 3D rectangular coordinate system with a slurry injection hole as an original point, designing a spatial position of monitoring points in an osmotic pressure monitoring network and determining a 3D coordinate of each monitoring points, arranging a high-precision infrared osmometer in the designed monitoring points, sending data monitored by the infrared osmometer to a data processing terminal, and drawing a slurry injection hydraulic pressure and slurry diffusing range spatial distribution map according to the data achieved by the data processing terminal. The beneficial effects of the injection slurry diffusion range and rule determining method during anchor rod slurry injection are that compared with previous a theoretical model experiment for slurry diffusion search, deficiencies can be overcome that a method achieved in the theoretical experiment for determining a slurry diffusion range cannot be totally suitable for composite and variable geological conditions.