As a major inorganic nitrogen source，ammonium can not only be absorbed by plant roots from the soil，but also generated during a number of biochemical processes inside plants. However，high concentrations of ammonium are toxic to plants resulting in growth inhibition and yield loss， therefore ammonium homeostasis needs to be under fine-tuned modulation in plants. Since currently nitrogen fertilizers are widely and largely used in the agricultural production， plants are facing the challenge of high ammonium presence. It is very important to understand the mechanisms of plant tolerance to ammonium toxicity for improving the use efficiency of nitrogen and the yield of crops. In order to explore the mechanisms of plant tolerance to ammonium toxicity， we reviewed the progress in mechanisms of plant tolerance to ammonium toxicity from three aspects including the source of ammonium nitrogen in plants，the causes of ammonium toxicity，and the physiological and molecular mechanisms of plant tolerance to ammonium. It was pointed out that the acidification of rhizosphere and apoplast caused by the absorption and assimilation of ammonium was a major cause of ammonium toxicity. Plants can alleviate the ammonium toxicity through their own mechanisms and the addition of other nutrients. At the same time， the research on mechanisms of plant tolerance to ammonium toxicity in the future was prospected. It will provide the theoretical reference for the molecular breeding of crops to tolerate ammonium toxicity and utilize nitrogen efficiently.