The work aims to apply in the papermaking industry the white rosin size prepared in the laboratory as paper sizing agent. White rosin size was prepared in the saponification reaction method and the method used for even stirring with high-speed mixer. The sizing effect of the sizing agent on the waste paper deinked pulp was examined. The results showed that the added white rosin size could improve the water resistance of the paper sheets and the quantity of absorbed water of the sheet was decreased from 283.33 g/m2 (for unsized deinked pulp) to 34.17 g/m2. Compared with unsized waste paper deinked pulp, tensile index and tear index of sheets applied with white rosin size had increased by 18.8% and 2.4% respectively. In addition, the strength performance of the sheets had also been improved. The addition of white rosin size self-made in laboratory can improve the water resistance of paper sheets made from waste paper deinked pulp and play the role in enhancing the strength of the sheets.