The invention belongs to the field of mortar, and particularly discloses phase change energy storage mortar and a preparation method thereof. The mortar is composed of, by weight, 180-200 parts of cement, 20-30 parts of fly ash, 20-30 parts of desulfurization ash, 680-760 parts of sand, 40-60 parts of a composite phase change material, and 110-130 parts of water. The preparation method includes: adding the cement, fly ash, desulfurization ash and sand into a mortar stirring machine for dry-mixing the raw materials for 30 s; after the raw materials are mixed uniformly, adding the composite phase change material, and stirring the mixture and meanwhile adding water, and stirring the mixture for 3 min. The mortar has high mechanical performance and latent heat storage effect, and overcomes theproblems of resource shortage of conventional cement mixtures, such as fly ash, and limited recycling approaches and low utilization rate of the desulfurization ash.