Supernova remnants (SNR) have long been considered as the major sources of Galactic cosmic rays (CRs). The standard shock acceleration theory predicts SNRs of fast shock, which are usually younger than a few hundred years old, can accelerate protons up to PeV. However, there is few evidence supporting acceleration of PeV CR protons in SNRs. Recently, we found the presence of X-ray synchrotron radiation from SNR G106.3+2.7. It implies a high-speed shock operating in this SNR despite it is likely already at the middle-aged stage of about 10\,kyr. Combining the previous study on the atomic cloud and molecular cloud distribution in this region and the multiwavelength spectral energy distribution with our X-ray analysis, we suggest that SNR G106.3+2.7 is likely a proton PeVatron.