This is a prospective, multicenter, open-label, phase III randomized controlled clinical trial designed to evaluate the efficacy and safety of replacing the traditional chemotherapeutic drug mitomycin with the PD-1 inhibitor sintilimab in definitive chemoradiotherapy for limited-stage anal squamous cell carcinoma. The study plans to enroll 350 previously untreated patients with limited-stage anal squamous cell carcinoma and randomize them in a 1:1 ratio into two groups: the control group will receive the current standard treatment, namely intensity-modulated radiotherapy (IMRT) concurrent with capecitabine and mitomycin; the experimental group will receive an innovative "immunotherapy replacement" regimen, namely IMRT of the same technique concurrent with capecitabine and sintilimab. The study adopts a dual primary endpoint design, aiming to verify that the experimental group is non-inferior to the control group in the clinical complete response rate at 6 months after radiotherapy, and is significantly superior to the control group in the incidence of grade 3 or higher treatment-related acute toxicities.