Methylmalonic acidemia (MMA) is a common organic acidemia that is autosomal recessive and results from reduced activity of methylmalonyl coenzyme A or defective metabolism of its coenzyme cobalamin, which is also a coenzyme for methionine synthesis from homocysteine. Because they do not affect methylcobalamin activity, patients with methylmalonic acidemia only, with normal homocysteine levels, are called simple methylmalonic acidemia. The gene encoding methylmalonyl coenzyme A allelase is MUT (MIM 251000) gene, which is located at autosome 6p12.3 and contains 13 exons. More than 200 mutations of MUT gene have been reported abroad, most of which are missense mutations. The treatment of simplex methylmalonic acidemia mainly includes restricting the intake of natural protein and supplementing with special milk powder and L-carnitine; some patients who are effective in vitamin B12 treatment are treated with vitamin B12 intramuscular injection. In recent years, foreign literature has reported a reduction in mortality and an improved prognosis in patients with simplex methylmalonic acidemia, but long-term complications are still unavoidable, and no large sample studies have been reported in China. In summary, the outcome and prognosis of patients with simplex methylmalonic acidemia are related to the type of disease, age of onset, and responsiveness to vitamin B12 treatment, and the prognosis of those with neonatal onset and ineffective vitamin B12 treatment is poor. Early diagnosis and early treatment are beneficial to the treatment and prognosis of patients.