A closed-form model of multiphoton quantum radar cross-section (QRCS) in the monostatic scenes is constructed for rectangular flat plates based on quantum interference and uncertainty. The model is justified by the comprehensive analysis of the model parameters in the model building process. Then, we use the model to quantitatively analyze the main lobe enhancement effect of multiphoton QRCS, which means that the more incident photons will enhance the main lobe magnitude of QRCS with other factors being the same. Moreover, we predict that enhancement effects might also exist for the side lobe close to the main lobe. In addition, we present the specific conditions for side lobe enhancement. On this basis, the enhancement angle range is defined to unify the description of the main lobe and side lobe enhancement effects. The influencing factors of the enhancement angle range are clarified. The results exhibit that the angle range of enhancement in multiphoton QRCS fluctuates with the change of target size and incident wavelength. All enhancement effects are exponentially related to the incident photon number. This work brings the description of multiphoton QRCS into the closed-form model analysis stage, which will provide prior information for research in many fields, such as photonic technology, radar technology, and precision metrology.