The measurement of dihedral angle is one of the most important measuring tasks in geometric measurement. Numerous and diverse angle measuring methods are proposed. However, few in-situ and automatic measuring methods are seriously and vigorously studied. To this end, a novel and simple angle measuring method is proposed, which is based on non-contact rotary ray-scanning mechanism. The rotary rays, emitted from a laser displacement sensor rotates around a fixed axis, constitutes the kernel part of the measuring probe. Which is driven to scan the surface of the workpiece. The distances from the target planes to the probe are obtained and subsequent data analysis is applied to calculate the dihedral angle. As an optional accessory, the measurement probe can be installed on the spindle of numerical control machine for in-situ measurements. The detailed mathematical model as well as the associated tolerance analysis are presented and validating experiments are offered. The method can do well in angle measuring within a nominal measuring range from 0° to 120°. The results are compared against the relative true value offered by three-coordinate measuring machine. It is shown that the maximal error of measuring result is 14″, and relative error of the proposed approach can be lower than 0.048% even in in-situ complex measuring environments.