The throat area of the geometry-variable nozzle directly affects the performance of the aero-engine. The inner diameter micrometer is traditionally used to measure the throat size, which has some problems, such as low efficiency and droop of the nozzle seal affecting the measurement accuracy. Therefore, we put forward a calibration method for throat area of tunable circular nozzle based on laser interference. Firstly, a coaxial length measurement structure is designed which features built-in laser interference optical path. A pressure sensor is utilized to give real-time feedback to automatically control the thrust. Then the polygon throat area of the nozzle is calculated from the dimensional measurement data. Finally, the correlation model between throat area and engine control signal is established based on the least square method. The experimental result shows that the size measurement accuracy of the developed system is 41 μm, the area calibration accuracy is 0.98147, and the method features good simplicity and convenience.