A novel cationic asphalt emulsifier of N-(3-polyoxyethylene nonyl phenyl ether (10)-2-hydroxyl) propyl-N,N,N-triethyl ammonium chloride was synthesized by nonylphenol polyoxyethylene ether (NP-10), epichlorohydrin and triethylamine. The optimum reaction condition was obtained. The yield reaches 48.72% at the optimum conditions of the feedstock mole ratio of NaOH to NP-10 1.0, the mole ratio of epichlorohydrin to NP-10 1.2, the mole ratio of triethylamine to NP-10 1.2, reaction temperature 70°C, and reaction time 8 h. The structure of the emulsifier was identified by FTIR. The synthesis process was monitored by online FTIR technique and the intermediate was detected. Based upon the experimental data, a plausible reaction mechanism was proposed for the reaction. The emulsifier belongs to slow-set asphalt emulsifier.