The maximizing iso-paraffins (MIP) process is a new fluid catalytic cracking route to produce cleaner gasoline. Its major innovation is the diameter-transformed fluidized bed reactor which can be flexibly regulated to multiple distinct reaction zones. This study aims to accurately reveal complex behaviors in MIP reactor by two-phase modeling and three-phase modeling to extend its application. Both simulations of an industrial MIP reactor are compared in terms of solids and liquid concentration, temperature, coke content, gas velocity, and product yield. It is found the two-phase case is enough for predicting the region far away from the oil feedstock injection, and the other is the better choice especially in the first zone if the heat transfer model can be reasonably built.