A new type of melamine-based polyether polyol (HMMM-PG) was synthesized based on 1,2-propylene glycol (PG) and hexamethoxy methylene melamine (HMMM). The structure and properties of HMMM-PG were characterized using IR, LC-MS, TG, and shear viscosity analysis, which showed that HMMM-PG had better thermal stability than common polyols as well as shear thinning rheological characteristics. In addition, HMMM-PGwas used to prepare polyurethane foams. The compressive strength, thermal conductivity, TG, SEM, LOI, HoC, and XPS were investigated to study the physical-mechanical and fire-retardant properties of the foams. The results showed that the physical-mechanical properties of the foams using HMMM-PG were substantially improved, which was attributed to the higher degree of hydroxyl functionality, which increased the crosslinking density and the number of triazine rings in the structures. Meanwhile, the flame-retardant properties of foams could be significantly improved, which were mainly reflected in the condensed phase. The more stable compounds were retained in the carbon residues during combustion. The continuous and dense char layer was formed. This char layer was effective in preventing heat transfer, and hindered the spread of decomposition products to the flame region.