An automatic voice informing device used for automatically informing a person of a to-do thing by voice is composed of a sensing and detecting part, a voice system, a control system, a power part and attachments. The sensing and detecting part, the voice system and the control system are mounted on a circuit board and are integrally fixed in a casing with the power part, and the casing is fixed in the middle part of a frame column. When a person goes out and opens a door, a to-do thing is automatically detected and sensed and the person is automatically informed of the to-do thing by voice, the specific contents of the to-do thing can be changed freely, and the person enters the door without hearing any prompt. The device is reasonable in design, simple in structure and convenient to operate, a prompt is given by voice as long as the person goes out and opens the door, the problem of key forgetting is completely solved, and the device is low in cost and convenient to install and has high application values.