The invention relates to an outdoor mobile robot guiding method and system. The method comprises steps that (1), the current longitude, the latitude and a heading angle of a robot are calculated; (2),whether the robot receives the path planning result is determined, that is, a target guiding path coordinate point set; (3), a guiding route is updated to obtain a guiding straight line function; (4), lateral deviation deltad and angular deviation deltatheta are calculated according to the longitude, the latitude, the heading angle and the guiding straight line function; (5), the deltad and the deltatheta are taken as feedback observations inputted into a digital PID controller to obtain the control quantity which is taken as the input quantity of a servo motor, and the mobile robot is drivento move according to a guiding route; (6), the current longitude, the latitude and the heading angle of the computing robot are updated; (7), whether the robot moves to an intermediate point is determined; (8), whether the robot reaches a final target point is determined, if yes, the process ends, and robot guiding is completed. The method is advantaged in that the angular error can be corrected,lateral deviation can be corrected, and the method not only adapts to wide roads and sites, but also adapts to narrow roads and environments.