The invention provides a preparation method of a concrete internal curing agent. The preparation method comprises the following steps: mixing acrylamide, a hydrophobic monomer, ethanol, a crosslinking agent and an initiating agent uniformly, heating until the temperature is 50-70 DEG C, then carrying out reaction for 4-10 hours, and carrying out conventional after treatment on a generated gel to obtain a shell material with a hydrophobic structure; mixing deionized water, acrylic acid and sodium hydroxide uniformly, controlling the neutralization temperature to be lower than 30 DEG C, then sequentially adding a crosslinking agent and an initiating agent, and mixing uniformly to obtain a core solution; and soaking the shell material with the hydrophobic structure in the core solution for 30-60 minutes, putting the shell material after filtration into a drying oven for reaction for 1-3 hours at 60-80 DEG C, and finally carrying out conventional after treatment to obtain the concrete internal curing agent. According to the preparation method, the preparation process is simple and convenient, the chemical structure is easy to control, an outer layer hydrophobic structure of the prepared internal curing agent enables the water absorbing rate to be relatively slow and can significantly reduce the influence of the water absorbing rate on the workability of concrete, and an inner layer hydrophilic structure can improve the early autogenous shrinkage reduction effect to solve the difficult problem of concrete early cracking.