The invention relates to a compound flux of high-MgO sinter ore, and a preparation method and application of the compound flux, belongs to the field of hematite sintering. The compound flux comprises a MgO source and a water-soluble additive, wherein the water-soluble additive is a combination of one or more than one of boric anhydride, calcium chloride, ferric oxalate, iron nitrate and ammonium ferrie oxalate. The preparation method comprises the following steps: first, dissolving the water-soluble additive in water; then, pouring the MgO source material to realize that the additive is uniformly adhered on the surfaces of the MgO source particles; putting into a drying oven below the temperature of 95 DEG C for drying, thereby obtaining the compound flux of the high-MgO sinter ore. During sintering, the compound flux, the hematite and the lime flux are mixed together to serve as the MgO flux, so that the MgO content in the sinter ore can be increased to 4 percent. Meanwhile, the additive has a low melting point, the sintering mineralization rate is improved, and as a result, reasonable mineral composition and phase structure can be obtained, the sinter ore strength and high-temperature metallurgical property can be improved, also, the finished product ratio and drum rotating index are obviously improved.