With the swift advancement of aerospace science and technology, the accurate measurement of coating thickness is particularly important, which is directly related to the performance and safety of aircraft.In this paper, a terahertz nondestructive testing method of coating thickness based on a standard tool was developed for the coating thickness measurement in aerospace field,by using a quartz window,the time-of-flight change caused by material thickness into the refractive index change caused by equivalent air thickness,the total thickness and refractive index of the composite coating can be accurately measured without prior knowledge.The results shows that the thickness measurement accuracy of this method can reach 3μm for single dielectric material and 5μm for coating material,also can accurately obtain the equivalent refractive index of the sample in terahertz band.The aforementioned technology can effectively detect the thickness and refractive index of composite coatings with high precision, making it highly valuable for precise coating thickness control applications such as aerospace.