Context. Recently, HAWC, AS gamma, and LHAASO experiments have reported the gamma-ray spectrum of supernova remnant (SNR) G106.3+2.7 above 40 TeV, indicating that SNR G106.3+2.7 is a promising PeVatron candidate. However, the origin of the gamma-ray spectrum is still debated. Thus, a dedicated theoretical model with self-consistent descriptions is required to decipher the properties of the gamma-ray spectrum for this specific source. Aims. We construct a theoretical model to explain the multiband photon emission from the PeVatron SNR G106.3+2.7. Methods. In our model, the acceleration and propagation of particles from the Bohm-like diffusion region inside the SNR to the Galactic diffusion region outside the SNR are described through nonlinear diffusive shock acceleration (NLDSA). The main content of our NLDSA model is solving the hydrodynamic equations numerically for gas density, gas velocity, and gas pressure and the equation for the quasi-isotropic particle momentum distribution. The consequent multiband nonthermal emission stems from two different regions, namely the acceleration region and the escaping region. Results. Our model is capable of explaining the multiband photon emission via the dominant synchrotron radiation of the electrons accelerated inside the SNR. The photons with energy of greater than or similar to GeV are naturally produced by the protons inside and outside the SNR. Moreover, photons in the energy range of similar to 1- similar to 100 TeV are due to the interaction of escaped protons with dense molecular clouds. Conclusions. For photons with energy E gamma greater than or similar to 1 GeV from SNR G106.3+2.7, our results here favor a hadronic origin, where the photons in the energy range of similar to 1G eV to similar to 1 TeV are produced inside the SNR through proton-proton interaction, while photons with E gamma greater than or similar to TeV originate from the interaction of escaped protons with a dense molecular cloud.