A distance measuring device includes a motor, a control box and a code discs which are relative rotate driven by the motor. A point position tooth is comprised on the code disc. The control box comprises a distance measuring unit, a detection part and a control unit. The detection part comprises a light emitter and a light receiver which are correspondingly arranged. The control box is rotated relative to the code disc, so that the point position tooth passes through a corresponding position between the light emitter and the light receiver; the control unit receives the signal output of the light receiver, judges the information on alignment status of the point position tooth with the corresponding position, and sends a start or stop operation instruction to the distance measuring unit on the basis of the status information. A method for seeking a distance measuring starting point is also provided.