Polyester　type　of　waterborne　polyurethane　emulsions　with　different　contents　of　2,　2-dimethylolpropionic　acid　(DMPA)　were　prepared　from　polyester　polyol,　toluene-2,　4-diisocyanate　and　DMPA　by　prepolymerization.　Effect　of　addition　amount　of　DMPA,　used　as　hydrophilic　chain　extending　agent,　on　stability,　particle　size,　kinematic　viscosity　and　anti-electrolyte　ability　of　emulsions,　and　water　absorption　and　mechanical　properties　of　membranes　obtained　from　emulsions　were　investigated.　The　membranes　were　characterized　by　means　of　WAXD　and　thermal　gravity　analysis.　Along　with　the　increase　of　DMPA　content,　stability,　kinematic　viscosity　and　anti-electrolyte　ability　of　polyurethane　emulsions　improved,　but　the　particle　size　decreased.　Similarly　tensile　strength　and　structural　order　of　relevant　membranes　improved,　but　water　resistance,　elongation　at　break　and　thermal　stability　decreased.　Suitable　mass　fraction　of　DMPA　is　7%-8%.