Abstract: Thermal cracking and hydrogen transfer reactions occurred in FCC naphtha cracking for propylene were studied on pilot riser FCC unit. Influence of operation parameters (reaction temperature, catalyst to oil ratio, resident time) on propylene selectivity, thermal reaction and hydrogen transfer reactions were investigated and thermal cracking reaction and hydrogen transfer reactions were discussed at maximal value of propylene selectivity. The result showed proper reaction temperature, catalyst to oil ratio and short reaction time may detriment thermal cracking reaction, hydrogen transfer reactions , and get high propylene selectivity.