The invention provides a structure for realizing external arc measurement based on a profile measuring instrument. The structure comprises a profile measuring instrument measuring rod, a first measuring head, a spring, a thread adjusting tube and a second measuring head. The movable first measuring head penetrates the front end of the profile measuring instrument measuring rod; an intermediate baffle is connected to the middle of the first measuring head and a spring sleeves the first measuring head; and the spring is arranged between the intermediate baffle and the profile measuring instrument measuring rod. The thread adjusting tube penetrates the rear end of a horizontal connecting tube; and the front end of the horizontal connecting tube is connected with a second measuring head. Whena rotating clamping disc drives a spline housing part to make rotation, the first measuring head and the second measuring head measure the external arc and the internal arc of a spline housing part respectively and heights of the second measuring head and the horizontal connecting tube at the thread adjusting tube can be adjusted. A clamping nut at the thread adjusting tube is used for fixation and thus the second measuring head is attached to the internal arc of the spline housing part.