Hydraulic conductivity of soil decreases significantly after MICP treatment, but there is a lack of theoretical calculation of the hydraulic conductivity of MICP-treated soil. This paper presents theoretical analysis and experimental study on hydraulic conductivity of MICP treated granite residual soils, and proposes theoretical expressions for CaCO₃ contents and hydraulic conductivity. CaCO₃ mass contents were derived based on the kinetic equation of the enzymatic reaction that decays linearly. Moreover, the particle size and number of CaCO₃ crystals were calculated using the SEM image, and then void ratio, tortuosity and average specific surface area of the grain were deduced. These factors were substituted into Kozeny-Carman equation to propose a theoretical expression of the hydraulic conductivity of MICP-treated soils. Compared to the experimental data, the results show that CaCO₃ contents increase sharply at the beginning and then become stable, while hydraulic conductivity exhibits the features of an early rapid decrease and subsequent tendency to stabilize. Hydraulic conductivity of the specimens with the concentration of 0.50 kmol/m³, 0.75 kmol/m³, 1.00 kmol/m³ and 1.25 kmol/m³ of cementitious solution decreased by 35%, 40%, 45% and 55% respectively. The expression of CaCO₃ mass contents and the model of hydraulic conductivity agree well with the testing data. The findings provided valuable insights into prediction of CaCO₃ contents and hydraulic conductivity of MICP treated granite residual soil.