Microencapsulated phase change material (mPCM) slurry has the advantages of excellent flow and heat transfer performance as traditional single-phase fluids and high heat storage density as phase change materials (PCM). In this paper, a three-dimensional numerical model of an immersed heat storage tank containing a vertical helical coiled tube was established. Combined with the physical properties of the prepared mPCM slurry, the coupling relationship between temperature and velocity field and the mPCM slurry’s natural convective heat transfer process were simulated. The influence of particle concentrations on its heat transfer and storage performances was discussed. Results show that the heat transfer process of mPCM slurry can be divided into three stages: heat conduction dominated stage-convection development stage-convection attenuation stage. Although the increase in particle concentrations reduces the heat transfer coefficient, the heat storage capacity of mPCM slurry with 30% particle concentrations increases by 38.6% compared with base fluid (water).