Abstract: The mechanical properties of basalt are crucial for future marine resource extraction, lunar exploration, and Mars base development. However, obtaining intact basalt cores is challenging due to the difficulties of sampling in deep-sea and deep-space environments, making it impractical to determine their mechanical properties using traditional macroscopic rock mechanics tests. In this study, a new method to assess the mechanical properties of non-standard basalt specimens based on micro-rock mechanics experiments and accurate grain-based modeling (AGBM) was proposed. The TESCAN Integrated Mineral Analyzer (TIMA) was used to analyze the mineral composition and microstructure of basalt. Nanoindentation tests determined the elastic moduli of rock-forming minerals. AGBM model of basalt was constructed based on digital images obtained by TIMA and mechanical parameters derived from nanoindentation. It is found that the elastic modulus of basalt obtained through AGBM-based numerical simulation of uniaxial compression closely aligns with those from macroscopic experiments. In contrast, homogenization methods, including the Voigt-Reuss-Hill scheme, the Mori-Tanaka scheme, and dilution scheme, show considerable discrepancies. Finally, the study examines the impacts of interphase mechanical properties, porosity, and pore filling on the upscaling results of the AGBM model. The proposed method provides an approach for predicting the mechanical properties of basalt samples in arbitrary shapes and small sizes.