When the filling mining method is used for two-step stope mining, the stability of the stope structure depends on the exposure area of the side wall when both sides are filling bodies and the strength of the filling body is constant. In order to determine the reasonable side wall exposure area under different filling body strengths, the Mathews stability diagram method is used to study the exposure area for reasonable side wall with one-step filling body during two-step mining. By plotting the stability number and shape factor on the stability diagram, the probability of the regression equation is taken as the proportion of instances in the process from mine stability to destruction and collapse. The stability number, shape factor, strength factor, joint direction adjustment parameter, and gravity adjustment factor coefficient are analyzed respectively. Based on the Q system classification, several stability diagrams are used to determine the shape factor, thereby estimating the exposure area value of the side wall of the filling body. According to the strength of the different one-step filling body corresponding to different stapes of the Makeng iron ore, the Mathews stability diagram method is analyzed, and the allowable lateral exposure areas of the filling body are 3600m²,5800m² and 6800m² respectively under the three different strengths of 1.75MPa,2.35MPa and 2.70MPa . The rationality of the exposure area of the one-step filling body determined based on the Mathews stability diagram method is further verified by the two-step stope blasting and mining of the Makeng iron ore. The results can provide referenc for the selection of reasonable lateral exposure area of the filling body during the two-step mining of mines using the subsequent filling mining method.