The invention aims to provides a prediction method for an interdendritic shrinkage porosity defect of a directionally solidified casting piece, and is characterized in that a criterion function G/R is adopted for predicting interdendritic shrinkage porosity distribution in the directionally solidified casting piece, wherein G (DEG C/mm) and R (mm/min) respectively represent a temperature gradient and a casting piece solidification rate in a casting piece solid-liquid interface forefront liquid phase in the directional solidification process; when the criterion function G/R of a certain region in the directionally solidified casting piece solidification process is less than or equal to C, the region of the directionally solidified casting piece can generate the interdendritic shrinkage porosity defect, wherein C is a constant, and the value of C is related to an alloy composition used in the casting. The method can realize the numerical simulation prediction of the interdendritic shrinkage porosity defect in the directionally solidified casting piece, and provides favorable conditions for shortening a development cycle of the directionally solidified casting piece.