A multi die edge-defined film-fed growth system was designed to simultaneously grow six sapphire ribbons in a crucible, to increase single run throughput. The thermal field in the system must be precisely tailored to create conditions for stable growth of multiple crystals. A fully coupled 3D global model, including thermal conduction, thermal radiation, melt convection and gas flow, was established to investigate the thermal field in the system. On the basis of the simulation results, we proposed two methods to improve the thermal field and further simulations were carried out to validate their feasibility. The results indicate that both a reduction in heater height and the use of dies with differing heights are feasible for stable multiple crystal growth, although both have some disadvantages. (C) 2013 Elsevier B.V. All rights reserved.