The utility model discloses a sand-adding device for coal mine underground fracturing or slotting. The device comprises a water inlet pipe, a mixing pipe and a water outlet pipe which are connected in sequence. A first electric valve, a first digital pressure gauge, a first electromagnetic flowmeter are arranged on a pipeline of the water inlet pipe. An outlet end of the water outlet pipe is used for being connected with a medium receiving port of a medium booster pump group. A check valve, a second digital pressure gauge and a second electromagnetic flowmeter are arranged on a pipeline of the water outlet pipe. A hopper used for containing sand materials is communicated with the upper portion of the mixing pipe through a sand feeding pipe. An electric discharge valve and an electric gate valve are arranged on a pipeline of the sand feeding pipe. The first electric valve, the electric discharge valve and the electric gate valve are all in an explosion proof type. The water inlet pipe, the mixing pipe and the water outlet pipe are respectively fixed on a flat plate mine car. The sand-adding device for coal mine underground fracturing or slotting has the advantages of being simple in structure, small in size and convenient to convey and transfer when the device is moved in or out of a well, sand-adding labor intensity is lowered, amount of used labor is lowered, potential safety hazards are reduced, and coal layer slotting efficiency is improved or permeability-increasing effect of coal and rock mass after fracturing is guaranteed.