The internal pressure within fission gas bubbles (FGBs) in irradiated nuclear fuels drives mechanical interactions with the surrounding fuel skeleton. To investigate the micromechanical stress fields in irradiated nuclear fuels containing pressurized FGBs, a mechanical constitutive model for the equivalent solid of FGBs was developed and validated. This model was based on the modified Van der Waals equation, incorporating the effects of surface tension. Using this model, the micromechanical fields in irradiated U-10Mo fuels with randomly distributed FGBs were calculated during uniaxial tensile testing via the finite element (FE) method. The macroscopic elastic constants of the irradiated U-10Mo fuels were then derived using homogenization theory, and the influences of bubble pressure, bubble size, and porosity on these constants were examined. Results show that adjacent FGBs exhibit mechanical interactions, which leads to distinct stress concentrations in the surrounding fuel skeleton. The macroscopic elastic constants of irradiated U-10Mo fuels decrease with increasing the macroscopic porosity, which can be quantitatively described by the Mori-Tanaka model. In contrast, bubble pressure and size have negligible effects on these constants.