<p>The hydro-mechanical behavior of grain-structured rocks, such as granite, is significantly influenced by the heterogeneity arising from the mineral grains and their boundaries. This paper presents a novel hydro-mechanical phase field model that incorporates the influence of mineral grains on the hydraulic fracturing process in such rocks. The model integrates the unified phase field fracture theory and Biot's theory, and utilizes Voronoi tessellations to represent the mineral grains. An interface phase-field method is proposed to capture the weakened strength at grain boundaries. Numerical examples compared with analytical solutions validate the model's effectiveness and demonstrate its capability to simulate complex hydro-mechanical fracture behaviors in grain-structured rocks. The results highlight the significant influence of mineral grain distribution on hydraulically induced fracture propagation paths, leading to the formation of intergranular and transgranular cracks and resulting in tortuous crack paths. The proposed model offers a valuable tool for understanding and predicting hydro-mechanical fracture processes in grain-structured rocks, and provides insights into similar models involving multiphase materials and potential engineering applications.</p>