In this article, for a better understanding of the viscosity changes in LPMC (low pressure sheet molding compounds), the thickening mechanism of crystalline polyesters was studied. And by means of varying the content of crystalline polyester resin, the thickening effect on resin paste was investigated, too. Results gained during experiments showed that there existed three kinds of functions acting in the process of thickening: swelling, hydrogen bonds and induction crystallization. The thickening effect was excellent when the amount of crystalline polyester resin was 3%. This research supports the producing process for LPMC with theory basement.