In order to maintain the fluid loss additive properties and the solubility at the same time，allyl methacrylate，acrylic acid， butyl acrylate were introduced during the vinyl acetate copolymerization process，a lightly branched polyvinyl alcohol（B-PVA）was prepared. The structure of B-PVA was characterized by FTIR，1H-NMR and DSC，and the solubility and the filtration-reducing ability of the B-PBA were investigated. FTIR and 1H-NMR spectra analysis proved that the structure of B-PVA was in accord with that of the object product，and the DSC results showed that the melting point of B-PVA was lower than that of PVA 1799，because the introduction of copolymerization units prevented the hydrogen interaction between hydroxyl groups in some extent. The B-PVA had the same solubility as PVA 1799；however，the B-PVA had a better filtration-reducing property compared to that of PVA 1799. Under the same condition，when the temperature raised up from 70℃ to 90℃，the filtrate loss of the slurry with PVA 1799 increased dramatically from 30 mL to 400 mL；However，the filtrate loss of the slurry with B-PVA only increased from 36 mL to 70 mL. The polymer B-PVA has potential use in oil exploitation.