The invention relates to a fixing method of a wall surface plate and a fixing device needed by the fixing method. The heat-preserving and decoration integrated swallow tail tenon and mortise fixing device is provided with fastening screws and a swallow tail pressing bar. A cross section of the swallow tail pressing bar is symmetrical leftwards and rightwards. A central axis of the swallow tail pressing bar is provided with multiple equidistant through holes. The fastening screws can be installed in the through holes. The cross section of the swallow tail pressing bar is a door-shape, a U-shape, an isosceles trapezoid, a half round or an isosceles triangle. A spacing distance between the through holes is 3-30 cm, and preferably 5-20 cm. An included angle Alpha between a pressing leg on theswallow tail pressing bar and the upper end face is 30-80 degrees, preferably 50-70 degrees, and more preferably 65 degrees. The pressing leg has elasticity. The tail end of the pressing leg is cut tobe flat or a semicircle oval shape. While the tail end of the pressing leg is cut to be flat, the tail end of the pressing leg is parallel to the upper end face of the swallow tail pressing bar. Theswallow tail pressing bar is thermoplastic plastic or metal.