Recently a novel multi-container extrusion method has been proposed and proved feasible to simultaneously extrude multiple billets for producing thin-walled wide profiles with reduced force by experiments. Due to its different die structure compared to conventional extrusion methods, the effects of tooling geometries on the multi-container extrusion performance should be comprehensively analysed as the base of die design optimisation. In this study, the original three-container extrusion design and six modified designs were established to investigate the influence of three key geometrical variables, including container diameter, upper die height and welding chamber height, on the extrusion performance through finite element simulation. The considered extrusion performances include metal flow uniformity, extrusion force, welding pressure, die lifetime and material yield. The drawbacks of the original die design were revealed after the primary analysis of the key influencing variables and a three-step way of design improvement was proposed accordingly. Container block and upper die were optimised for the first step; welding chamber height and die bearing length were modified during the second and third steps respectively. Compared with the original design, the final optimised design can decrease the unevenness of the extrudate front shape by 75% and double the material yield, while the extrusion force, die lifetime and welding quality were basically unchanged.