Low pressure sheet molding compound (LPMC, 1.0-3.0 MPa, 95-103 ℃) is a new kind of thermosetting material with crystalline polyester as a physical thickenner. LPMC is different from conventional SMC using an earth oxide thickening agent (e.g. MgO) as chemical thickenner, it relies on the physical thickening of crystalline polyester. Crystalline polyester resin is the key material to mold LPMC parts. Currently there was no report about the thickening mechanism of crystalline polyester in LPMC. In this article, crystalline polyester resins, whose melting points were between 45 ℃ and 89 ℃, were synthesized by a two-step esterification. The melt points of crystalline polyesters are controlled by regulating the mol ratio of the two glycols and the two acids. And by means of varying the content of crystalline polyester resin, the thickening effect on resin paste is investigated. In addition, the thickening mechanism of crystalline polyester in LPMC was investigated by FTIR and DSC analysis. The effects of the diameters and viscosity of crystalline polyester on the rheological property and fiber distribution of LPMC sheets were studied, too. Results show that the thickening effect is excellent when the weight content of crystalline polyester resin is 3%. And there exists three kinds of functions acting in the process of thickening: swelling, hydrogen bonds and induction crystallization. During the preparing process of resin paste in LPMC, the temperature of resin paste must be kept at 90 ℃. In addition, crystalline polyester make LPMC have a perfect fluid property. When the viscosity of LPMC sheet is beyond 1 kPa·s, the fiber orientation is not obvious. But when the viscosity of LPMC sheet is about 500 Pa·s, the fiber shows a certain degree of orientation. Moreover the study of physical and chemical thickening mechanism of crystalline polyester and the rheological discipline of LPMC sheets in the hot mould will provide the researchers and enterprises with theory guidance.