To improve the imaging quality of the geometric waveguide, a method of measuring the parallelism of the waveguide array based on the off-axis autocollimation optical path was proposed. In this study, the error evaluation model of the parallelism of the semi-transparent and semi-reflective film array in the waveguide was analyzed according to the characteristics of the array plane. Combining the imaging principle of the geometric waveguide with the autocollimation optical path, the mathematical relationship of the array parallelism was deduced. Further, the parallelism measurement system of the waveguide array was developed. The system utilized a light source controller to control the output light intensity, which prevented the measurement from being affected by the light energy utilization of different semi-transparent and semi-reflective films in the waveguide. Subsequently, the Steger algorithm was used to process the autocollimation image. Finally, the calibration, array parallelism, and verification experiments were conducted. The experimental results show that the measurement uncertainty of the parallelism measurement system is 1.14″, and the maximum repeatability error is 0.32″. The proposed method can quickly and accurately measure the parallelism of the geometric waveguide array, which is of guiding significance to the attitude correction of the waveguide array and imaging quality improvement.