OBJECTIVE: To investigate the classification and incidence of acid-base disturbance (ABD) in the patients with post-traumatic multiple organ dysfunction syndrome (MODS). METHODS: A total of 119 patients with MODS were examined with arterial blood gas analysis and serum electrolytes detection for 675 times in this study. RESULTS: Different types of ABD existed in 647 times out of 675 times (95.9%) of blood-gas analyses. There were 270 times (41.7%) of simple ABD, 271 times (41.9%) of double ABD and 106 times (16.4%) of triple ABD. Among which, 404 times (62.4%) were in respiratory alkalosis (RAL), 332 times (51.3%) in metabolic acidosis (MA), 227 times (35.1%) in metabolic alkalosis (MAL) and 167 times (25.8%) in respiratory acidosis (RA). In this study, 79 cases (66.4%) out of 119 cases with MODS died from these kinds of ABD. CONCLUSIONS: It suggests that in the early stage of MODS, RAL with or without hypoxemia may exist, and later on, MA or even triple ABD may occur. In order to detect and correct the primary disorders as early as possible, it is important to keep the balance of hydrolyte. The treatment of primary diseases is also important. Disorders of acid-base balance were corrected according to pH standard values, anion gap (AG) and the potential [HCO(3)(-)] were also calculated simultaneously. When pH was more than 7.50 or lower than 7.20, it is necessary to give drugs of acidity or alkalinity to the patients with ABD to maintain pH value within a normal range.