A novel water-soluble binder used in photoimagible solder mask is synthesized and its performance is studied with infrared spectroscopy, gel-permeation chromatography, photosensitivity guide, and optical microscopy. The binders with different molecular weights and acid values are synthesized by varying the monomer concentration, the initiator content, and the polymerization temperature. It is concluded that the binder with a weight average molecular weight at 80000~90000 and an acid value near 130~140 exhibits the good photosensitivity and resolution. (Keywords: water-soluble, photosensitivity, solder mask, binder, and synthesis)