Thermogravimetry experiments and thermal analysis kinetics are used to analyze the effects of magnesium hydroxide on asphalt pyrolysis characteristics. The pyrolysis models are developed based on experiments to evaluate the flame-retarded mechanism of magnesium hydroxide to asphalt. The results indicate that asphalt pyrolysis process in N2 is a single stage, and presents unimodal character in DTG curves. Owing to endothermic decomposition and crystal water release of magnesium hydroxide, the temperature rise of asphalt is inhibited. Pyrolysis of SBS modified asphalt follows a one-dimensional pyrolysis mechanism model, but flame-retarded SBS modified asphalt follows a three-dimensional pyrolysis mechanism model. After adding magnesium hydroxide, asphalt pyrolysis kinetics parameters are raised and become available to decrease reactive activity and rate. Based on this study, it is obvious that the thermal stability of asphalt is improved by magnesium hydroxide under high temperatures. Equally, the flame-retarded performance of asphalt is also enhanced by using magnesium hydroxide.