Abstract: In situ tumor vaccine has become an important strategy in cancer immunotherapy owing to its ability to induce immune responses locally and overcome tumor heterogeneity. However, the abnormal structure and mechanical properties of the tumor’s physical microenvironment significantly limit the efficiency of vaccine delivery and immune efficacy. In this review, the key factors in the tumor’s physical microenvironment, including solid pressure, interstitial fluid pressure, matrix stiffness, and tissue microstructure, are systematically discussed. Their obstructive roles in immune cell infiltration, antigen presentation, and immune activation are analyzed. The potential of approaches, such as radiotherapy, anti-angiogenic therapy, extracellular matrix degradation agents, nanomaterials, and hydrogel delivery platforms, in reshaping the tumor’s physical microenvironment is explored. This review aims to offer theoretical and practical guidance for optimizing in situ vaccine strategies through the regulation of the tumor’s physical microenvironment, ultimately advancing the precision and effectiveness of cancer immunotherapy.