Based on study on the formation conditions of overthrust and its seal mechanism, this paper considered that the main factors influencing vertical seal of overthrust depended on deformation characteristics of plastic rocks accompaning overthrust, pressure exerted on fault zone and mudstone content in fault zone. A simulation experiment on the quantitative relation between mudstone content in fault zone and vertical seal of overthrust was carried out. In addition, this paper analyzed the influence of deformation characteristics of rocks on vertical seal of fault. The evaluation standard for vertical seal of fault was established which put forward a method evaluating vertical seal of fault by the mudstone content in fault zone, the pressure exerted on fault plane and the deformation characteristics of gypsum bearing mudstone. Taking the Kela 3 Structure in the Kuqa Depression as an example, the vertical seal of fault was studied, and the results indicated the method being reliable.