The invention discloses a vehicle-mounted positioning system and an automatic driving vehicle. One specific implementation manner of the system comprises a sensor assembly, an automobile computer and man-machine interaction equipment, wherein the sensor assembly is used for acquiring external environment information, position information and attitude information of the automatic driving vehicle; the automobile computer is used for receiving the external environment information, the position information and the attitude information from the sensor assembly, determining absolute positioning information of the automatic driving vehicle at the current moment based on the external environment information, the position information and a high-precision map, carrying out motion compensation based on the external environment information and the attitude information to obtain relative positioning information of the automatic driving vehicle at the current moment, and fusing the absolute positioning information and the relative positioning information to obtain self-positioning information of the automatic driving vehicle at the current moment; the man-machine interaction equipment is used for representing the self-positioning information. By adopting the implementation manner, the precision of the self-positioning information is improved and the accuracy of the self-positioning information, which is output when communication of the system is temporarily stopped or hindered, is improved.