Plantar pressure data contain rich information on human gait and posture. To achieve convenient and accurate measurement of ground reaction force (GRF) during walking, this article proposes a method for measuring human GRF based on wearable inertial measurement units (IMUs). This method utilizes information from three IMUs located on the human trunk and both feet to achieve segmentation of walking gait cycles and estimation of the GRF magnitude on both feet. To verify the effectiveness of this method, walking data from six subjects with significant differences in body parameters from a public dataset were tested. The test results at four walking speeds showed that the average RMSE for the six subjects were 34.34 ± 4.88 N, 34.87 ± 5.66 N, 43.50 ± 3.73 N, 41.36 ± 3.02 N, 42.01 ± 6.39 N, and 37.02 ± 5.06 N. This indicates that the method presented in this article can accurately predict the GRF on both legs.