The invention discloses a three-dimensional parameter extraction method of an internal defect of a gas turbine blade based on a digital ray. The method comprises the steps of arranging two-dimensional detection images of the defect according to pixels for primary finite element division, then performing discrete quantification on thickness of each pixel according to gray-scale values, determining a corresponding relation between grey-scale and the thickness, accumulating finite elements of all the pixels, and extracting a three-dimensional parameter of a defect area.The method is based on a finite element concept; the two-dimensional detection images of the defect are arranged according to the pixel for the primary finite element division; the discrete quantification is performed on the thickness of each pixel according to the gray-scale value; and further, the corresponding relation between the grey-scale and the thickness is determined.The three-dimensional parameter of the defect is extracted by accumulating finite element areas of all the pixels; the deficiency of the traditional radiographic detection method in three-dimensional parameter extraction of the defect can be effectively made up for; and the extraction of the three-dimensional parameter of the internal defect of the gas turbine blade can be achieved more efficiently at lower cost.