In this work, a novel sulfur-based flame retardant (SA-M) was synthesized by the self-assembly of melamine and sulfamic acid. The chemical structure of SA-M was fully characterized. SA-M, in company with Al2O3, was then introduced into polyamide 11 (PA 11) by melt compounding in order to improve the fire resistance of the polymer substrate. The observation by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) indicated the well dispersion of SA-M in PA 11 matrix. The fire performance of PA 11 composites was evaluated by limiting oxygen index (LOI), vertical burning (UL-94), and cone calorimeter tests, respectively. The results showed that the presence of 17.5% SA-M and 2.5% Al2O3 increased the LOI value from 22.4% to 30.9%, upgraded the UL-94 rating from no rating to V-0, significantly eliminated the melt dripping, and decreased the peak heat release rate from 1024 to 603 kW/m2. The thermal behaviors were investigated by thermogravimeric analysis (TGA) and TGA-Fourier transform infrared spectroscopy (FTIR). It was suggested that SA-M took effects mainly in gas phase by diluting the combustible fuel, leading to the improvement of the fire resistance of PA 11.