The invention discloses a cast-in-situ construction method for a reinforced concrete structural system taking phosphogypsum as a wall body material. When construction is performed on a reinforced concrete structural system building, the phosphogypsum is used as the wall body material, the reinforced concrete structural system and phosphogypsum wall bodies are cast in situ. When the reinforced concrete structural system and the phosphogypsum wall bodies are cast in situ, the reinforced concrete structural system is cast in four sections according to floor layers; the height of each section is controlled within 600 to 1,300 millimeters; the cast-in-situ construction is performed in layers and sections from the top to bottom; the reinforced concrete structural system is cast first at each section; and then the phosphogypsum wall body part is cast. The construction method has the advantages of easy construction, fast construction progress, good construction quality and low construction cost. In addition, the integrity and strength of buildings completed by the construction method are improved.