Abstract: Taking methane/ethylene (Me/E) ratio and meth-ane/propylene (Me/P) ratio as the control indexes of cracking level of heavy oil contact cracking process, the influences of process conditions, such as reaction temperature, feeding airspeed and water - to - oil ratio, on cracking level are studied on experimental units on fixed fluid beds. Experiment results show that the cracking level goes up along with reaction temperature increasing and feeding airspeed lowering; while the water- to- oil ratio has insignificant impacts on cracking level, though it can improve the distribution of products. Suitable cracking level indexes should be Me/E ranging from 0. 35 t o0. 36 and Me/P from 0. 55 to 0. 60.