The ``$\pi^0$-decay bump" discovered in the gamma-ray spectra of some interacting supernova remnants (SNRs) is regarded as direct evidence for SNR acceleration of cosmic rays (CRs). However, the ability of SNRs to accelerate CRs to sub-PeV or even PeV energies remains a topic of ongoing debate. We report the LHAASO gamma-ray spectrum measurement of W51C, an interacting SNR exhibiting a $\pi^0$-decay bump. Owing to the unprecedentedly high sensitivity of LHAASO, we extend the gamma-ray spectrum to $\approx200$~TeV, an order of magnitude higher than the prior measurements. The results indicate a cutoff energy of $\approx400$~TeV for the incident proton spectrum, providing strong evidence that SNRs can serve as accelerators for sub-PeV CRs.