The invention relates to esomeprazole sodium and a preparation method thereof. The preparation method comprises the following steps: S1, preparing an esomeprazole sodium water solution; S2, preparingan oil-phase organic solution; S3, mixing the esomeprazole sodium water solution prepared in the step S1, the oil-phase organic solution prepared in the step S2 and a sodium alginate water solution, and performing ultrasonic emulsification, thereby obtaining a mixed-phase emulsion containing sodium alginate; S4, enabling the mixed-phase emulsion containing sodium alginate prepared in the step S3 to react with a calcium chloride water solution to obtain slow-release esomeprazole sodium; and S5, performing washing, filtering and freeze drying on the slow-release esomeprazole sodium obtained in the step S4. The preparation method has the advantages of capability of ensuring high stability, small particle size, uniform grain distribution and high drug loading degree of esomeprazole sodium andhigh drug slow release performance.