The invention discloses a method for preparing cement mortar. The method comprises the following steps of: after drying a selected raw material, namely trimethylolpropane (TMP) for a certain time, reacting the TMP with toluene diisocyanate under a certain condition to prepare a prepolymer; after filtering methoxyl polyethylene glycol under a certain condition, reacting the methoxyl polyethylene glycol with the prepolymer under a certain condition to prepare high-performance water soluble polyurethane; after cooling, diluting the water soluble polyurethane by adding water according to certain solid content; and stirring the prepared water soluble polyurethane and cement, fine sand, an additive and the like according to a certain weight ratio by a specific stirring process to obtain the high-performance cement mortar. The cement mortar prepared by the method has the advantages of high breaking strength, high elastic modulus and high corrosion resistance.