The invention discloses a track mileage positioning method and device based on a CPIII control point, and belongs to the field of railway track measurement. The track mileage positioning method and device are used for determining the track mileage corresponding to the position where a track inspection trolley is positioned. The implementation mode comprises the steps that the first distance between a first reference point on the track inspection trolley and the target CPIII control point and the second distance between a second reference point on the track inspection trolley and the target CPIII control point are measured, and the third distance between the first reference point and the second reference point is known; according to the first distance, the second distance and the third distance, the relative position relation of a first projection point, a second projection point and a third projection point of the first reference point, the second reference point and the target CPIII control point on the track center line is obtained; and based on the relative position relation of the first projection point, the second projection point and the third projection point, the track mileage of the first projection point and the track mileage of the second projection point are obtained, and then the track mileage value of the position, corresponding to the center line, of the track inspection trolley is obtained by combining the geometrical parameters of the track inspection trolley. The mode of measuring the mileage is simple and practical.