Due to the condensation reaction of hydroxylated melamine molecules is possible to take place in the whole pH range, especially in acidic water medium or at high temperature, losing its water solubility. In this work, freeze drying perfectly removes water at pretty low temperature, avoiding condensation reaction of hydroxylated melamine. Therefore, a hydroxylated melamine in solid form (MOH‐S) was prepared. It can be not only dissolved in water after storing for 180 days but also well dispersed in waterborne polyurethane emulsion. The addition of hydroxylated melamine significantly improves the tensile strength of polyurethane films by 170%. After soaking the films in water for 24 h, the tensile strength of WPU/30%MOH‐S film lost only 6%, but 34% loss for the neat. Fourier transform infrared spectroscopy peak fitting demonstrates that the H‐bonding degree of N─H in WPU/30%MOH‐S film is 20.72 that is much higher than 3.33 in neat WPU. This indicates that H‐bonding cross‐linked structure is formed between polyurethane chains and hydroxylated melamine molecules. This work provides an effective strategy for preparing stable hydroxylated melamine and enhancing the strength of polyurethane film.