Ethylene-co-vinyl acetate (EVA) is widely used in daily life and the development of its detection technology is very important. In this study, the thermal gravimetric analysis coupled with mass spectrometry (TGA-MS) was applied to develop a method for the direct quantitative analysis of EVA without further sample pretreatment. The degradation of EVA as measured by the combination of TGA with Fourier transform infrared spectrometer (FTIR) and gas chromatograph-mass spectrometer (GC/MS) consists of three steps. The results show that a large amount of acetic acid produced in the deacetylation step of second stage can be clearly identified, while (CH)n fragments and acetone produced in this step. With the increase of pyrolysis temperature, benzene derivatives and large (CH2) fragments are detected. This indicates the chain scission reactions occurring at the end of the polymer main chain during deacetylation. Isothermal deacetylation reaction is monitored by MS which determines that the formation of ketones does not affect deacetylation. MS signal intensities of acetic acid linearly responding to EVA concentrations, therefore the quantitative analysis of EVA can be clearly identified by acetic acid. This method is easier operation and rapid to test the concentration of EVA, at the same time it can be a viable complement to existing methods.