The invention discloses a braking sail derailment device for a cube satellite. The braking sail derailment device comprises a locking device, a storage mechanism, a mounting panel, a conical spring, an unfolding mechanism and a diaphragm sail. The locking device is fixed on the top face of the mounting panel. The storage mechanism is fixed on the bottom face of the mounting panel. The conical spring, the unfolding mechanism and the diaphragm sail are arranged in the storage mechanism. The large-diameter end of the conical spring is fixedly connected with the mounting panel, and the small-diameter end of the conical spring is fixedly connected with the unfolding mechanism. The diaphragm sail is tied to the unfolding mechanism. The braking sail derailment device is fixedly connected to the bottom of the satellite through the mounting panel at the top, so that the space inside the satellite is not occupied. After receiving a command from the ground, the locking device releases a central shaft in the unfolding mechanism, and a strap elastic mast wound on the central shaft drives the diaphragm sail fixed on the mast to be unfolded by releasing elastic potential stored in the mast. According to the invention, the diaphragm sail is unfolded to increase the cross-sectional area in the flight direction of the cube satellite, and the atmospheric resistance of the cube satellite is increased, so that the cube satellite derails quickly.