A method for preparing a fiber-reinforced composite article initially performs a trial molding by a molding machine to prepare a trial composite article of a composite molding material including a polymeric material having a plurality of fibers, wherein the trial composite article has a trial fiber orientation distribution. The method further generates a predicted fiber orientation distribution fitting with the trial fiber orientation distribution, wherein the predicted fiber orientation distribution is generated by performing a first molding simulation for the trial composite article by using physical rheology parameters and physical fiber orientation parameters. The method further performs a second molding simulation for a real composite article by using the physical rheology parameters and the physical fiber orientation parameters to obtain molding conditions for the molding machine, and performs a real molding process by the molding machine by using the molding conditions to prepare the real composite article.