Te invention discloses a method for growing a large-size rubidium titanyl phosphate monocrystalline by using a hydrothermal method. The method specially comprises the following step: in an autoclave, by taking a RbxH(3-x)PO4 (x is greater than or equal to 1 but less than or equal to 3) with rubidium ion concentration of 0.5-5.5mol/L as a mineralizer, dissolving rubidium titanyl phosphate by adopting a temperature difference hydrothermal method and recrystallizing to grow the monocrystalline. Compared with an existing hydrothermal method, the method disclosed by the invention can be used for quickly and safely growing the large-size rubidium titanyl phosphate monocrystalline with intact crystal and high optical quality at low temperature and pressure; the obtained crystal can be used for electro-optical devices; the process is simpler and easier to control and lower in cost. Compared with an existing molten-salt growth method, the method is low in growth temperature and low in system viscosity, hardly introduces impurities due to adoption of an airtight environment, and can be used for obtaining a Y-directional device with the maximum electro-optic coefficient of the crystal. The yield is high.