Suitable structure for plenum chamber lead to a significant enhancement of the relaxation heat setting machine performance, this paper presents the influence of opening ratio and static pressure on flow velocity. Optimization design method is employed to identify the optimum opening ratio and structure dimension of plenum chamber quantitatively. Furthermore, a numerical approach is presented to validate the optimization results, numerical studies indicate that decreasing opening ratio has better effect on mending air flow uniformity than constant opening ratio, the optimized structure results in the relative velocity error rate decrease by 23.7%.