Provided are a method and a process for preparing MHP via controlled crystallization in hydrometallurgical processing of laterite nickel ore, belonging to the technical field of laterite nickel ore hydrometallurgy. The method comprises: determining a nuclei count of multiple crystal nuclei after nucleation reaction of a solution after iron and aluminum removal, based on nucleation reaction conditions; determining an average particle size of multiple crystal grains after primary growth of the multiple crystal nuclei, based on primary growth reaction conditions; and determining a predicted particle size distribution of agglomerated crystal grains after secondary agglomeration and growth of the multiple crystal grains, based on the Metropolis algorithm, the nuclei count, and the average particle size. According to the predicted particle size distribution results, the present application adjusts reaction conditions for the nucleation reaction and primary growth reaction to control the nuclei count and average particle size, thereby obtaining high-performance MHP products with optimized particle size distribution, providing guiding significance for MHP crystallization processes.