The invention discloses an apparatus for measuring the effective hole diameter of a deformation monitoring hole and the operation method thereof. The apparatus comprises a detection mechanism and a control bench, wherein the detection mechanism comprises supporting legs, a laser range finder, a gauge rod and a measurement apparatus body. The measurement apparatus body is connected with the control bench through a line, the supporting legs are connected to the sidewall of the measurement apparatus body through threads, and the laser range finder is installed on the center of the measurement apparatus body through the gauge rod. A built-in weight and compass of a probe allow the probe to move along the vertical direction and rotate along 360-degree direction, and the data measured by the laser range finder is transmitted to the control bench through the gauge rod and a cable conductor. The control bench comprises an operating system, a data acquisition and conversion system and a data display system. The operating system comprises a rising button, a decreasing button, a probe rotation button and a scanning button. The data acquisition and conversion system records scanning data and converts the scanning data into graphic data. The apparatus can be used to effectively check hole diameter, and can detect the quality of a deformation monitoring hole.