To realize autonomous positioning of mobile robot in floor, studying on the combined positioning, which included relative positioning based on encoder and gyroscope, absolute positioning of corridor combined with map information based on ultrasonic sensor and absolute positioning of target room position based on machine vision. This paper designed and realized the combined positioning system, to meet the working requirements, completed the overall plan design of positioning, relative position design of corridor, absolute positioning design of corridor intersection and the target doorplate. The appropriate sensor was selected and the verification experiment of the combined positioning system was carried out based on the wheeled mobile robot of laboratory. The experimental results showed that the positioning error of the system is not more than 30cm, and the positioning error at the target location is not more than 15cm. The combined positioning system can realize accurate positioning in floor.