The invention relates to the middle-convex profile modification gear tooth shape measurement field and provides a transparent detection plate ruler and a measurement method thereof. On an organic glass plate, according to a design drawing detection proportion, lines are scribed, and five point positions of an involute expansion height are given out. Through taking a middle-convex profile modification amount 0.005mm-0.02mm as an example, average four vertical lines between the 0.005mm-0.02mm are measured and scribed lines are a concave line groove structure. A tooth shape detection template possesses a tooth shape tolerance zone curve scope, an engagement range curve scope, an evaluation scope and the like. A detection-point scribed line graph theoretically accords with the design drawing and is superposed and compared with an actual involute tooth shape graph report so as to find an area with a maximum deviation. Each part number is used in a customization mode, which is taken as a main basis for detection.