Thermal cracking and secondary reactions occurred in FCC process were studied by using a continuous pilot riser-type FCC unit. The effects of operating parameters(reaction temperature, catalyst to oil ratio, contact time)on thermal cracking and secondary cracking were also investigated. Moreover, the influence of overcracking operation on thermal cracking, hydrogen transfer and isomerization reactions was particularly discussed. The experimental results show that low reaction temperature, short contact time of oil and catalyst, and high catalyst to oil ratio may suppress the detrimental thermal cracking dramatically, and benefit to hydrogen transfer and the isomerization reactions. In the overcracking operation caused by high temperature, the thermal cracking may be enhanced, and hydrogen transfer and the isomerization reactions may be decreased remarkably, while the condensation reaction between olefin and aromatic hydrocarbons can be enhanced. In the overcracking operation caused by high catalyst to oil ratio, the thermal cracking may be restrained while the hydrogen transfer and the isomerization reaction in the secondary reactions can be promoted obviously. In the overcracking operation caused by long contact time, the thermal cracking reaction increases and the hydrogen transfer and the isomerization reaction appear the same tendency as thermal cracking reaction.