Due to its safety, stability, low cost and easy installation, anchor plate support structure has been widely used in the slope that needs to be reinforced. However, currently in the design process of anchor plate support structure, there is no mature calculation method for the size of anchor plate, resulting in overly conservative design. To address this issue, we propose using the limit equilibrium method in combination with log-spiral slip surface to calculate the active earth pressure in unsaturated soils. Based on the internal balance of the soil, the tensile force required to bear for each layer of anchor plate is calculated, and the calculation method for the size of the anchor plate is obtained. The minimum design length of the free section of the anchor plate is obtained according to the shape of the sliding surface. The calculation results of active earth pressure are compared with the existing literature, and it is found that the change trend of active earth pressure coefficient is consistent with the existing conclusions. The impacts of rainfall and earthquake conditions on the size of anchor plate are analyzed, and the results show that low rainfall intensity has a negligible effect on the size of anchor plate. However, when considering both strong rainfall and earthquake, the required anchor plate size can be significantly increased, with a maximum of 136.71%. The analysis results can provide reference for determining the size of frame anchor plates.