<p>While silane coupling agents (SCAs) are widely used to modify the hydrophobicity of cementitious materials, it’s influence on the mixed-mode fracture resistance of calcium sulfoaluminate (CSA) systems is still unclear. Hence, the mixed-mode fracture performance of CSA by methyltrimethoxysilane (MTMS) is studied from the chemical transformations and microstructure shaping perspective. The results show that, in the case of sample without MTMS, the needle-like morphology of AFt is prominent and the AH₃ gel can fill the internal pores, while, with increasing MTMS content, AFt formation is suppressed, AH₃ gel becomes porous and reduced. At 2 and 5% MTMS dosage, internal defects in the CSA cement paste induce stress concentration, thereby reducing its tensile fracture resistance. Nonetheless, a 3% MTMS content promotes the formation of a dense pore structure within the paste, thereby increasing tensile fracture performance. The formation of Si–O–T and O–Na–O rock bridges contributes to improved shear fracture resistance of the paste. Furthermore, an extended curing age has a consistently positive impact on the tensile fracture of the pastes across all MTMS contents, with a particularly significant enhancement in shear performance observed at 2 and 5% dosages.</p>