The interface transition zone (ITZ) between the fiber and the cementitious matrix significantly influences the strengthening and toughening effect of the fiber on the matrix. This paper presents the microscopic characteristics of ITZ between magnesium-phosphate-cement (MPC) and steel fiber. The micromechanical properties and micromorphology of ITZ were assessed using Nano-indentation and Scanning Electron Microscopy, respectively. The effect of the proportion of MPC, the incorporation of silica fume (SF), curing time and types of cement (ordinary Portland cement, sulphoaluminate cement and MPC) on the microscopic characteristics of ITZ was experimentally investigated. The experimental results showed that the ITZ between steel fiber and MPC with P/M (mole ratio of potassium dihydrogen phosphate to magnesia) of 1/4 had the largest compactness, thinnest weak area, and highest micromechanical indices. The bonding performance between MPC and steel fiber was the optimal when P/M of 1/4. The incorporation of SF of 10% by weight significantly improved the compactness of ITZ, reduced the thickness of the weak area of ITZ, and increased the micromechanical indices of ITZ. As a result, the incorporation of SF of 10% by weight greatly improved the bonding performance between the MPC and steel fiber. The compactness and micromechanical indices of ITZ between steel fiber and MPC were the highest, compared to the ITZ between steel fiber and sulphoaluminate cement and between steel fiber and ordinary Portland cement. The experimental results presented in this study provide the basis for the application of steel fiber reinforced MPC-based concrete.