The invention provides a method for measuring the three-dimensional constrains of the existing line, which is characterized in that the operation method comprises the steps as follows: (1) a track constraining
pile is arranged along the railway track and can carry out simultaneous detection and adjustment to a three-dimensional measurement benchmark net of the existing line; (2) a track detection car with a track constrain
pile measurement function is parked at a track marking point corresponding to the track constrain
pile; and transverse offset and vertical offset between the
central line of the track and the track constrain pile are measured and calculated by a computer; (3) according to the coordinates of the track constrain pile and track line parameters, the practically measured coordinates of the track
central line are inversely calculated; and (4) according to the track line parameters, the transverse offset and the vertical offset between the practically measured coordinates of the track
central line and the theoretical coordinates of the central line are calculated and the sound and light alarm can be carried out according to the field overrun; and the method has the advantages that a
total station instrument is not required, and the whole
measurement device has compact structure, simple operation, high measurement efficiency, good adaptability for environment and climate conditions, low cost and meets the requirements of detection and measurement of
engineering affairs of the existing line.