The invention discloses a preparation method for a mixed desulfurizing agent for a circulating fluidized bed boiler, and relates to a desulfurizing technology in the circulating fluidized bed boiler. The preparation method solves the problems of low utilization rate of conventional limestone flour serving as a desulfurizing agent, high equipment loss and high power consumption. The preparation method comprises the technical steps of (1) conveying limestone fine-powder materials into a limestone powder bin; (2) stirring the limestone fine-powder materials by a star wheel feeder and a differential turbine so as to enable the limestone fine-powder materials to fall to a coal slime conveying belt; (3) adding coal slime into the conveying belt so as to be mixed with the limestone fine-powder materials; (4) stirring the coal slime and the limestone fine-powder material mixture in a double-shaft stirring machine, and then conveying to a briquette forming machine; (5) pressing the stirred coal slime and limestone fine-powder material mixture to form the spherical mixed desulfurizing agent; and (6) conveying the spherical mixed desulfurizing agent into a finished product bin for later use through a mixed desulfurizing agent conveying belt. The mixed desulfurizing agent prepared by the preparation method is used for realizing desulfurization in the circulating fluidized bed boiler and has the advantages of simple system, low investment, high desulfurizing agent utilization rate, low equipment loss and low power consumption.