Background: In recent years, some studies have shown that hyperphosphatemia and hypophosphatemia are associated with high mortality in intensive care unit (ICU) patients. Based on this, we speculated that the abnormalities of serum phosphate when patients enter the ICU had an adverse effect on the mortality of ICU patients and conducted our study.Methods: For our retrospective cohort study, we collected the data from a publicly accessible critical care database. We selected adult patients admitted to ICU with some inclusion criteria. And we extracted a large number of clinical variables. Using multivariate Cox regression and other statistical methods, we assessed the association between serum phosphate and primary endpoints.Results: Of 27131 eligible patients, the patients with hyperphosphatemia or hypophosphatemia had poorer clinical outcomes. After adjustment for potential confounders, there was no significant association between abnormalities phosphate and 28 or 90-day ICU mortality. Nevertheless, at the medical intensive care unit, hyperphosphatemia was associated with a 37% higher risk of 28-day mortality or a 25% higher risk of 90-day mortality.Conclusion: After adjustment for potential confounders, hypophosphatemia and hyperphosphatemia at ICU admission were not the independent risk factors of 28 or 90-day mortality for general ICU patients. However, hyperphosphatemia at admission increased the risk of 28 or 90-day mortality among patients admitted to the medical intensive care unit.