The invention provides a construction method for an extruded sheet outer wall external thermal insulation system. The construction method comprises that following steps that interface treating mortar is used for carrying out cleaning on an outer wall surface, and a heat preservation binding sizing agent is arranged; the heat preservation binding sizing agent is brushed on the outer wall surface; a polystyrene board is adhered on the heat preservation binding sizing agent; gridding cloth is laid on the polystyrene board; a high molecular bottom layer coating material is brushed on the gridding cloth; flexible crack-proof water-tolerant putty is brushed on the high molecular bottom layer coating material. The construction method has the advantages that compared with a traditional polystyrene board outer wall external thermal insulation system technology, due to the fact that the heat preservation binding sizing agent is used, the polystyrene board is adhered on the heat preservation binding sizing agent, the high molecular bottom layer coating material is brushed on the gridding cloth, and the flexible crack-proof water-tolerant putty is brushed on the high molecular bottom layer coating material, so that the better heat preservation effect can be achieved.