In this work, a low-melting-point chain extender (LMCE) was prepared using 1,4-butanediol (BDO) and dimethylol propionic acid (DMPA) as raw materials. A series of co-solvent-free waterborne polyurethanes were prepared based on LMCE. The structure and properties of the waterborne polyurethanes were characterized by gel permeation chromatography, infrared spectroscopy, particle size analysis, and tensile tests. It was found that the molecular weights of the WPU based on LMCE were much higher than that of the WPU based on DMPA. The increase of DMPA/BDO molar ratio led to smaller particle size and better micro-phase separation of the WPU. The results of mechanical test showed that the mechanical properties of the WPU decreased with decreasing of DMPA/BDO molar ratio. In addition, the effective cross-linking chain density of the WPU was evaluated based on the theory of rubber elasticity.