This is a multicenter, prospective, randomized phase II trial designed to evaluate the efficacy and safety of short-course neoadjuvant chemoradiotherapy combined with the PD-1 monoclonal antibody serplulimab and the anti-angiogenic agent bevacizumab in patients with previously untreated pMMR/MSS middle and low locally advanced rectal cancer. The study plans to enroll a total of 200 participants (100 in the experimental arm and 100 in the control arm).