The bubble pressure of the fission gas bubbles (FGBs) in irradiated nuclear fuels causes mechanical interaction between the FGBs and the surrounding fuel skeleton. To calculate the micromechanical stress fields of the irradiated nuclear fuels with pressured FGBs, an effective mechanical constitutive model for the FGBs is deduced and verified based on the modified Van der Waals equation and the effect of the surface tension. Based on the established model, the micromechanical fields of irradiated U-10Mo fuels with randomly distributed FGBs during the uniaxial tensile test are calculated by finite element (FE) method. The macroscopic elastic constants of the irradiated U-10Mo fuels are obtained according to the homogenization theory, and the effects of bubble pressure, bubble size, and porosity on the macroscopic elastic constants are investigated. The conclusions can be drawn that: (1) The bubble pressure of the FGBs is balanced by the constraint stress of the surface tension and the surrounding fuel skeleton. The constraint stress of the surrounding fuel skeleton is determined by its deformation states, not equal to the hydrostatic pressure of the surrounding fuel skeleton or that of the macroscopic material points of fuel elements; (2) Adjacent FGBs exist stress interference and are prone to result in stress concentration in the surrounding fuel skeleton; (3) The macroscopic elastic constants of irradiated U-10Mo fuels decrease with macroscopic porosity according with the Mori-Tanaka model, while bubble pressure and size have insignificant effects on the macroscopic elastic constants of irradiated U-10Mo fuels.