Leg cramps are one of the common symptoms during pregnancy. About 30%-50% of pregnant women experience leg cramps twice a week. Leg cramps may cause severe pain and sleep disturbance, hinder performance of daily activities and may lengthen the duration of pregnancy and the type of childbirth. Several randomized controlled trial (RCT) studies focused on the effects of the magnesium supplement for relieving leg cramps. However, the results were inconsistent. Five databases were searched from their inception to July 2, 2020. We summarized the weighted mean difference (WMD) with 95% CIs for "the frequency of leg cramps after treatment", and summarized the odds ratio (OR) with 95% confidence intervals (CIs) for "recovery from leg cramps" and "side effects". Four RCTs with a total of 332 pregnant women were identified. The frequency of leg cramps after treatment was not decreased in the treatment group compared to the control group (WMD = -0.47, 95% CI: -1.14-0.20, P = 0.167). Magnesium supplementation cannot improve the recovery from leg cramps compared to the control group (OR = 0.47, 95% CI: 0.14-1.52, P = 0.207). Magnesium supplementation had no significant side effects in the treatment group compared to the control group (OR = 1.82, 95% CI: 0.90-3.69, P = 0.094). Oral magnesium supplementation is not effective in the treatment of leg cramps during pregnancy. PROSPERO: CRD42020196572.