Focal mechanism solutions of microseismic events during Hot Dry Rock (HDR) hydraulic fracturing are critical parameters for revealing fracture propagation mechanisms. However, conventional methods struggle to address their characteristics of low energy, high event density, and complex genesis. Based on extensive microseismic data from HDR fracturing, this study develops a focal mechanism inversion method specifically for HDR microseismic events. By combining P-wave first-motion polarity, P/S amplitude ratios, and full-waveform information, a multi-constraint objective function is constructed to calculate the source moment tensor parameters. Finally, combined with iterative inversion of the in-situ stress field, the optimal fault plane is automatically determined, thus avoiding human-induced uncertainty. Applied to a carbonate-type HDR reservoir in eastern China, the method successfully calculated focal mechanisms for 145 microseismic events (M_W>-0.5). Practical application demonstrates that this framework enables efficient batch processing of microseismic focal mechanisms, providing reliable technical support for fracture characterization and fracturing optimization in HDR reservoirs.