The invention discloses a size-selective purification method for a salt water phase nanocrystal and relates to novel technology for purifying a water phase nanocrystal by using salt and the selective separation of water phase nanocrystals with different sizes by a salt purification method. The purification method comprises the following steps of: a, preparing appropriate saline solution; b, purifying the salt water phase nanocrystal; and c, performing size-selective purification on the salt water phase nanocrystal. The invention provides the size-selective purification method for the salt water phase nanocrystal aiming at the problem existing in conventional alcohol nanocrystal purification technology. The method has the advantages of simple operation, extremely low cost and wide application prospect in future nanocrystal industrial mass purification.