The invention discloses a zero-stress seam width self-adaption type roof reserved seam waterproof system and construction method of the zero-stress seam width self-adaption type roof reserved seam waterproof system, and belongs to the technical of roof water resistance. The system comprises a reserved seam waterproof sealing mechanism, a reserved seam roof embedded groove mechanism and a waterproof layer upper portion, wherein the reserved seam roof embedded groove mechanism comprises a roof lower layer, a reserved seam and embedded grooves; the embedded grooves are formed in the two sides of the reserved seam; the reserved seam waterproof sealing mechanism is made of low-elasticity mold materials and comprises an embedded component, a horizontal connection section, a vertical extending section and an elastic round ring; the section of the elastic round ring is a round ring with one open end; the two ends of the elastic round ring are connected with one end of the vertical extending section; the other end of the vertical extending section is connected with the embedded component through the horizontal connection section; the embedded component is embedded into the embedded grooves; the waterproof layer upper portion is located on the upper portion of the roof lower layer, the embedded component and the horizontal connection section. The reserved seam waterproof sealing mechanism is embedded into the roof material tightly and can be driven to stretch and deform along with the roof material, and self-adaption of the width of the reserved seam is achieved.