Reflective terahertz time-domain spectroscopy is employed to test the time-domain spectrum of coatings. The critical threshold to determine whether the coatings are waterborne or solvent is obtained by the ratio of the first reflection peak to the second reflection peak. This method can successfully achieve rapid analysis by placing the coatings in a measuring container without complicated sample pretreatment. The critical threshold will not be affected by disposable paper cups or polyethylene plastic containers, different testers or test time. This provides a fast and accurate method to identify waterborne coatings and solvent coatings in the field of environmental protection supervision.