The invention relates to an HDPE geomembrane and bentonite-clay composite vertical anti-seepage wall and a mounting construction method thereof. The HDPE geomembrane and bentonite-clay composite vertical anti-seepage wall is arranged on underground opposite water-resisting layers, and composed of an HDPE geomembrane and a bentonite-clay anti-seepage layer. The mounting construction method comprises the steps that a polluted area and a pollution control field area are subjected to engineering geology and hydrogeology survey; a site is leveled; the HDPE geomembrane is vertically inserted and laid; bentonite-clay slurry is intermittently replenished; and a trapezoidal dam integrally connected with a guide wall and a guide wall platform is formed. Through the technical scheme, the HDPE geomembrane and bentonite-clay composite vertical anti-seepage wall and the mounting construction method thereof have the advantages that mounting is easy, the cost is low, the anti-seepage layer structure is not damaged, and pollution is disposed in situ; and local materials can be used in the construction process, on the basis of meeting original anti-seepage requirements, the safety and stability of ageomembrane anti-seepage layer can be improved, and damage, caused by external environmental disturbance or the bottom jacking effect, to the geomembrane is avoided.