Objective To investigate the magnesium disorder and determine the effect of serum magnesium level on short-term prognosis in the old patients with acute kidney injury (AKI). Methods The old male patients(≥75 years old) with AKI admitted in the Geriatric Department of our hospital between January 2007 and December 2015 were enrolled in this study. According to their outcomes in 28 d and from 29 to 60 d after AKI respectively, they were also divided into survival and death groups. Hypomagnesemia is defined as serum magnesium <0.7 mmol/L, and hypermagnesemia as >1.1 mmol/L. SPSS statistics 17.0 was used to perform the statistical analysis. The survival curves were estimated by Kaplan-Meier (product-limit) Estimator, and compared by Mantel (log-rank) test. Multivariate Cox proportional regression model was used to analyze the effect of serum magnesium on short-term survival in elderly patients. Results There were 623 old male AKI patients included, at a median age of 87(84,91) years, including 473 patients (75.9%) in the normal magnesium, 72(11.6%) in hypomagnesemia, and 78(12.5%) in hypermagnesemia. One hundred and sixty patients (24.5%) died within 28 d after AKI, and 194 cases (29.8%) died within 60 d. The Kaplan-Meier survival curve showed the high magnesium patients had worse 28-day survival (log rank P=0.001). Multivariate Cox analysis revealed that time of AKI diagnosis (HR=0.865,5%CI 0.799-0.937; P<0.001), mean aortic pressure(HR=0.970, 95%CI 0.958-0.981; P<0.001), serum prealbumin level(HR=0.924,5%CI 0.894-0.955; P<0.001), oliguria (HR=2.261, 95%CI 1.424-3.590; P=0.001), mechanical ventilation(HR=1.492,5%CI 1.047-2.124; P=0.027), blood urea nitrogen level (HR=1.037, 95%CI 1.025-1.049; P<0.001), magnesium level (HR=2.512,5%CI 1.243-5.076; P=0.010) and AKI stages (stage 2:HR=3.709, 95%CI 1.926-7.141, P<0.001; stage 3:HR=5.660,5%CI 2.990-10.717, P<0.001) were the influencing factors for 28-day mortality. However, no significant difference was found in serum magnesium level for 29 to 60-day mortality. Conclusion The incidence of magnesium disorder is up to 24.1% in old male AKI patients. High serum magnesium level is associated with 28-day mortality, so more intensive monitoring and appropriate correction of serum magnesium level may prolong their survival time.