The invention discloses a GH4738 alloy fine-grain blank manufacturing method for a supercritical power station steam turbine blade and aims at existing technical states that a temperature field and the deformation rate are difficult to meet high requirements of alloy deformation parameter sensitivity degree, and microstructures are low in homogenization degree in a process of blank manufacturing by hammer forging a GH4738 alloy ingot. According to the method, an extrusion method for blank manufacturing is used for replacing an original hammer forging manner for blank breakdown. The GH4738 alloy ingot is heated to the recrystallization temperature or more so as to be subjected to extrusion deformation; the moulding of the alloy ingot can be improved by using a three-direction pressure state in the extrusion deformation, so that the moulding deformation can be played to the greatest extent; the fine grain is improved by adopting the deformation with a large extrusion ratio; a coating and sleeving technology is used for reducing the temperature drop during transfer and extrusion deformation of the alloy, so that the extrusion ratio is controlled accurately; a temperature rising effect is adopted for enabling the alloy to be subjected to heat deformation in the appropriate temperature field all the time, so that alloy bars having the fine grain and the high microstructure homogenization degree are manufactured.