The invention relates to a stable inorganic hydrated salt based phase change energy storage material and a preparation method thereof. The material comprises the following components in percentage by weight: 80 to 95% of inorganic energy storage material, 0.25 to 10% of nucleating agent, and 1 to 10% of physical phase-separation resisting agent. The preparation method comprises the following steps: adding an inorganic energy storage material and a nucleating agent into water according to a certain ratio, stirring in a water bath with a temperature of 40 to 60 DEG C until a uniform and transparent solution is formed; then adding a phase-separation resisting agent into the solution, and evenly stirring. Compared with the prior art, a novel physical phase-separation resisting agent is used to improve the circulation stability of an inorganic hydrated salt phase change material system, and the thermal performance of the provided material barely attenuates after 500 times of thermal circulation. The phase-separation resisting agent is user-friendly and extensively applicable, and is suitable for massive production.