The Eulerian-Eulerian approach was applied to simulate the flow behavior and catalytic cracking reactions in the riser reactors of two-stage riser fluid catalytic cracking (TSRFCC) technology. A k -epsilon - k(p) - epsilon(p) - Theta gas-solid turbulent flow model was used, which took account of the particle turbulence and the interaction of turbulence between gas and particle phases. A 14-lump kinetics model was used for simulating cracking reactions. The approach and model were validated with both experimental results and commercial data. The distributions of particle fraction volume and velocity, as well as product yields in the TSRFCC riser reactors were first analyzed. The simulations were then carried out for optimization studies to understand the influence of the operating conditions on the performance of commercial TSRFCC riser reactors. The model and results presented here are valuable for the design and optimization of TSRFCC technology. (C) 2009 Published by Elsevier Ltd.