The rumen is inhabited by a multitude of microorganisms that play an important role in the production of ammonia. Microbe-mediated amino acid deamination and non-protein nitrogen hydrolysis are the main pathways for ruminal ammonia production. Ammonia produced by rumen microorganisms acts in a feedback loop to affect the structure of microbial flora and the function of ruminal epithelium, thus affecting the ruminal fermentation and host health. This review summarizes the production of ammonia and the influence of ammonia on digestion and epithelial function in rumen, aiming to provide a reference to the follow-up related research.