Series of polyimide (PI)/mesoporous silica nanospheres (MSNs) nanocomposite films with different contents of MSNs were successfully prepared via a simple wet impregnation method. The morphologies, microstructures, mechanical properties, transmittance, and thermal properties of the prepared PI and the PI/MSNs nanocomposite films were investigated in detail. As a result, the thermal stability and mechanical performances of PI were obviously improved by incorporating MSNs into PI. The tensile stress and Young’s modulus of the nanocomposite film with 5 wt % MSNs were raised up to 97.65 MPa and 2220.06 MPa, which are greatly higher than the values of 82.51 MPa and 1440.86 MPa for the pure PI film. Experimental results confirmed that the designed polymerization tactic, which occurred in the pores of the MSNs, facilitated to enhance the mechanical and physical performances of the PI/MSNs nanocomposite films, and definitely induced better integration between organic matrix and inorganic nanofillers.