The invention discloses a preparation method of core-shell type silica sol. The preparation method comprises steps as follows: 1) a polymer aqueous solution and ammonia water are weighed and added to a flask, and the solution is stirred until a polymer is completely dissolved; 2) the polymer ammonia water solution is dropwise added to a flask containing a solvent at 25 DEG C, and quick stirring is performed during addition; 3) alkoxy silane is dropwise added for 1 h, and the solution reacts continuously at the reaction temperature of 25-30 DEG C for 4 h after addition; 4) after the reaction, excessive cation exchange resin is directly added for stirring treatment, and the core-shell type silica sol is prepared. The preparation method is simple, the PH of the prepared core-shell type silica sol ranges from 5 to 6, the silica sol is stable to store, the inner diameter of nanoparticles is larger, and the concentration of the silica sol can be increased through concentration.