The utility model provides a device for observing the soil forming rate of purple mother rock in situ. The device comprises an interface flow stopping tank, a soil containing cage and a collecting box. According to the main structure of the device, the interface flow stopping tank is a rectangular bounding wall a nylon mesh is laid on the inner bottom surface of the interface flow stopping tank, a thin soil layer is laid on the nylon mesh, the interface flow stopping tank is built on the surface of the purple mother rock to be observed, the soil containing cage is arranged on the interface flow stopping tank, the interface flow stopping tank is connected with the collecting box through a flow discharge pipe, the flow discharge pipe is arranged below the nylon mesh in the interface flow stopping tank and is connected with the collecting box, the collecting box is connected with a water discharge pipe, the soil containing cage is composed of a steel reinforcement cage, a bar-mat reinforcement, a nylon mesh and a handle, a whole piece of bar-mat reinforcement is laid on the bottom surface and the four side surfaces of the steel reinforcement cage, and the nylon mesh is laid on the surface of the bar-mat reinforcement. By adopting the device provided by the utility model, the disturbance to the soil at the upper layer of the purple mother rock can be reduced greatly, the hydrothermal condition of the mother rock and soil interface can be simulated to the greatest extent, and the accuracy and reliability of the soil forming rate observation data can be improved.