To study the mechanical properties of grouting body under microseismic loads, the grouting in red sandstone fissure with a fracture angle of 30° and a width of 4 mm was taken as the research object, and the uniaxial compression test was carried out by rock three-axis servo press at different strain rates (10^-5-5×10^-3 s^-1). Then, from the three aspects of energy principle, crack propagation mechanism, and final failure morphology, the influence law and mechanism of variable strain rate on the mechanical properties of grouting were analyzed. Research shows that with the increase of strain rate, the peak strength and elastic modulus of the grouting body increased, and the peak intensity and strain rate changed exponentially. The response of the grouting body affected by strain rate was divided into sensitive strain rate stage and slow strain rate stage, and the main difference was in the peak strength change rate and elastic modulus change rate. As the strain rate increased, the total energy of the grouting body increased. The compaction phase was the main phase affecting the mechanical properties of the grouting body under different strain rates. The main difference between the compaction phase in the sensitive strain rate stage and the slow strain rate stage was in the dispersion ratio, which further determined the number of cracks and the distribution area and its law. The dissipated energy density had a great influence on the failure area and the particle size distribution of the grouting body, where the greater the dissipated energy density was, the greater the proportion of large fragments was (the proportion of larger particle size increased gradually). The dissipated energy density in the slow strain rate stage was larger than that in the sensitive strain rate stage, so the fragments were larger than those in the sensitive strain rate stage. By analyzing the mechanical properties of the grouting in fractured zone under the influence of variable strain rate, the influence mechanism was obtained from the perspectives of energy theory and fractal theory.