Based on the working principle of the reaction microscope and the technical parameters employed in our setup, all the possible factors which affect the momentum resolutions of the recoil-ions and the electrons are analyzed. The resolution dependences on the parameters are discussed in detail. The target extension is critical to the momentum and energy resolutions. For electrons the energy resolution is sensitive to the momentum anti-parallel to the extraction electric field: the larger the momentum, the worse the resolution is. The results are applicable to both longitudinal and transverse reaction microscopes.