Aliphatic units (ethylene succinate) were incorporated into aromatic water-soluble polyesters (WSP) in order to improve their environmental protection and sizing performances. The environmental performance was measured in terms of biological oxygen demand for 5 days ($BOD_5$) and chemical oxygen demand (COD). Apparent and intrinsic viscosities, the adhesion to polyester fibers, glass transition temperature, and mechanical behavior of sizing films of WSP were evaluated for improving their sizing performance. The composition of copolymeric WSP was also investigated by $^1H$-NMR. A series of copolymeric WSP with a variation in molar ratio of aliphatic monomer (dimethyl succinate) to aromatic monomer (dimethyl terephthalate) from 10:90 to 40:60 was synthesized through a two-step reaction, transesterification and polycondensation. It was found that, by incorporating aliphatic units into the macromolecular chain of aromatic WSP, the environmental performance, apparent and intrinsic viscosities, and the toughness of sizing film were enhanced obviously. The adhesion to polyester fibers showed no sensitivity to the molar ratio of dimethyl succinate to dimethyl terephthalate when the ratio ranged from 10:90 to 30:70. In view of overall performance of the aliphatic-containing WSP sizes, the molar ratio should be 30:70. Based on the ratio, the WSP prepared showed better environmental and sizing performances.