N-doping strategy has been explored to enhance the activity of carbon nanozymes because the reconstructed electronic structures in N-doped carbon nanozymes are advantageous for the catalytic process. However, carbon nanozymes with high N content are still difficult to obtain due to the instability of N element under high calcination temperatures. To address this challenge, here we proposed a new N-doping strategy to fabricate highly active and specific peroxidase-like carbon nanozymes by using a high N-containing polymer (i.e., polyethylenimine (PEI)) as the N source and a natural clay mineral (i.e., montmorillonite (MMT)) as a template, respectively. We showed that the assembly of MMT with PEI protected N loss under high calcination temperatures and thus retained more catalytically active N sites. The mechanism study showed that the hydroxyl radical could be the key intermediate involved in the peroxidase-like catalysis. We then used the optimized carbon nanozyme with high and specific peroxidase-like activity (i.e., CP600-6) to detect H2O2, glucose, and ascorbic acid. Moreover, we successfully determined the total antioxidant capacity (TAC) in real samples including four commercial beverages, fresh orange juice, and three kinds of vitamin C tablets. The current study not only provides a new strategy for fabricating peroxidase-like nanozymes but also develops a facile TAC assay for future use in evaluation of antioxidant food quality and oxidative stress in healthcare.