An apparatus and method for growing nitride bulk single crystal, including an autoclave having a pre-growth zone and a growth zone. With control of the concentration of a saturated solution in a pre-growth chamber, the oversaturation reaction conditions for the overall process of growth of the nitride bulk single crystal can be regulated. By regulating the liquid level difference of the melt on an upper surface of a seed crystal, nucleation growth of N/Ga is preferentially performed on the surface of the seed crystal, which suppresses polycrystal formation at a gas-liquid interface and improves the growth rate of crystal and the utilization rate of raw materials.