The finite-discrete element method (FDEM) can be used to simulate brittle materials such as polycrystalline ice with specific geometric information. However, most previous studies treat ice as intact and nonporous, ignoring the effect of internal porosity. In this study, an FDEM model of polycrystalline ice with specific porosity is built by using the cohesive interface element and the method of randomly deleting elements. Comparison with experimental results confirms that the model can capture the strength properties and deformation patterns of polycrystalline ice. The fracture failure patterns and mechanical responses of ice specimens and their relationships with porosity are investigated by uniaxial compression tests and Brazilian splitting tests. The results show that with increasing porosity, the fracture failure patterns of the specimens in the uniaxial compression test evolve into three types: global shear failure, local shear failure and local tensile-shear failure. There is no obvious difference in the failure patterns of the specimens in the Brazilian splitting test. In addition, as the porosity increases, the material exhibits a transition from brittleness to ductility, and the porosity also affects the local fragmentation characteristics inside the polycrystalline ice, significantly weakening the strength of the specimens.