Abstract: 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 get mechanical properties of non-standard basalt specimens based on microscopic rock mechanics experiments and accurate grain-based modeling (AGBM) is proposed. The TESCAN integrated mineral analyzer (TIMA) is used to analyze mineral composition and microstructure of basalt. Nanoindentation tests are used to obtain elastic moduli of rock-forming minerals. AGBM models of basalt are constructed based on digital images obtained by TIMA and mechanical parameters derived from nanoindentation. It is found that elastic moduli of basalt obtained through AGBM-based numerical simulations of uniaxial compression tests closely align with those from macroscopic experiments. In contrast, homogenization methods, including the Voigt-Reuss-Hill scheme, the Mori-Tanaka scheme, and the dilution scheme, show considerable discrepancies. Finally, the study examines the impacts of interphase mechanical properties, porosity, and pore filling on upscaling results of AGBM models. The proposed method provides an approach for predicting mechanical properties of basalt samples in arbitrary shapes and small sizes.