The utility model discloses an open pit mine slope deformation test device under indoor simulated rainfall conditions. The device comprises a slope similarity simulation test module, Rain simulation module, Monitoring module, the side slope similarity simulation test module comprises a side slope heap; Model frame, the side slope heap is placed in the model frame; the rainfall simulation module comprises a rainfall water tank; wherein the water tank and the rainfall water tank are connected through the main water pipe, the rainfall water tank is arranged on the upper portion of the model frame, the monitoring module comprises monitoring drawing pins and a theodolite used for observing displacement changes of the monitoring drawing pins, the monitoring drawing pins are arranged on a side slope pile-up object, the rainfall water tank is provided with a groove for containing water, and a bottom plate of the groove is provided with a plurality of rainfall holes. The structure of the side slope is simulated, water is in a raindrop falling shape when flowing out of the rainfall holes, simulated rainfall is more real and reliable, side slope deformation under the rainfall condition can beideally simulated, and the side slope deformation process, the stress mechanism and factors influencing side slope stability are analyzed.