The present disclosure provides a method for operating a molding system. The molding system includes a molding machine and a mold disposed on the molding machine, wherein the mold has a mold cavity for being filled with a molding material from the molding machine. The method comprises a step of obtaining a predetermined state waveform expressing a predetermined volumetric variation of the molding material. Next, the method further comprises obtaining a measured pressure and a measured temperature of the molding material in the mold cavity while performing a molding process for filling the molding material into the mold cavity. Next, the method includes obtaining a detected volumetric property of the molding material corresponding to the measured pressure and the measured temperature. Subsequently, the method comprises displaying the detected volumetric property of the molding material with the predetermined state waveform.