The invention discloses a modified phosphogypsum cement thickening time control agent which includes phosphogypsum and a modifier and is prepared from the following components in percentage by mass: 75 to 95 percent of phosphogypsum, 5 to 15 percent of quick lime and 2.5 to 10 percent of anhydrous sodium sulphate. The phosphogypsum is comprehensively modified by compounding the phosphogypsum withquick lime and anhydrous sodium sulphate, the pH value of the modified phosphogypsum reaches 7 or above, and the content of soluble phosphorus is effectively reduced. The modified phosphogypsum is used for the cement retarder, the production process is simple, the problems that discharged phosphogypsum occupies land and pollutes the environment are solved, the win-win situation of economic benefits and social benefits can be achieved for cement production enterprises; and therefore, the modified phosphogypsum is the cement retarder worthy of application and popularization.