The invention belongs to the field of surface treatment of metals, and in particular relates to surface pretreatment solution and a pretreatment method used for bonding magnesium alloys. The surface pretreatment solution comprises the following components: 10 to 50 g/L dihydric phosphate, 1 to 5 g/L compound accelerant, and 0.5 to 3.0 g/L sodium fluoride, wherein the compound accelerant is formed by mixing molybdate, tungstate and nitrate. A phosphide film obtained by the pretreatment solution and the pretreatment method of the invention has the characteristics of completeness, uniformity and compactness, so a bonded joint has high shearing strength and very high corrosion resistance in a corrosion environment; and a phenomenon of the strength reduction of the joint is lightened. In addition, the environmental pollution caused by heavy metal ions such as hexavalent chrome in the pretreatment solution is eliminated, so the method is an environmental-friendly surface pretreatment method for bonding the magnesium alloys.