Abstract: The thickening agent of phenolic sheet molding compound and its thickening mechanism were studied in this paper.The thickening agent of phenolic sheet molding is investigated by the viscosity tests.The results show that the mixture of B₂O₃ and ZnO is the proper thickening agent of phenolic sheet molding compound and the suitable thickening effect can be achieved when 4 portion B₂O₃ and 1 portion ZnO are added into 100 portion phenolic resin.And at the same time,the infrared analysis is used to analyze the thickening mechanism of B₂O₃ and ZnO to the Phenolic resin,respectively.It is found that B₂O₃ has fine effect of thickening on the phenolic resin and ZnO not only has no effect on the resin but also delays the effect of B₂O₃ thickening on the resin.