A method for operating a molding machine includes steps of specifying a simulating domain corresponding to a genuine domain in a mold disposed on the molding machine; performing a virtual molding by using an initial packing pressure profile to generate an initial state waveform expressing a relationship between an in-mold pressure and an in-mold temperature of the molding resin; obtaining an updated packing pressure profile for applying a molding pressure to at least a portion of the genuine domain while taking into consideration the initial state waveform; repeating the virtual molding while taking into consideration the updated packing pressure profile to generate an updated state waveform of the molding resin; and setting the molding machine taking into consideration the updated packing pressure profile to perform an actual molding by applying an actual molding pressure to the at least a portion of the genuine domain to prepare the molding product.