The invention discloses an automatic feed supplementing straw smashing device. The automatic feed supplementing straw smashing device comprises a machine body, and the machine body is internally provided with a working cavity and a feed storage cavity and is provided with a feeding channel communicated with the working cavity and a feed putting opening which is communicated with the feed storage cavity; a window communicated with the feed storage cavity and the working cavity is formed between the feed storage cavity and the working cavity; a feeding platform fixed to the machine body is arranged at the end, away from the working cavity, of the feeding channel, and the feeding channel is internally provided with a feeding mechanism; the working cavity is internally provided with a smashingmechanism, wherein the window is provided with a lidding mechanism capable of controlling the window to open/close according to the feed amount in the working cavity; the feed storage cavity is internally provided with a horizontal rotary shaft, a driving mechanism is connected to the rotary shaft, and several radially fixed rake teeth are distributed on the circumference of the rotary shaft; thedriving mechanism drives the rotary shaft to rotate in a window opening state. It can be guaranteed that the device is in a full load state in the working process all the time, and feed can also be effectively prevented against clogging in the window.