Blow molding heads are widely used in the polymer processing for packaging. However, it is difficult to optimize the blow molding heads because of the complexity of flow distribution with regard to problems of head geometry, temperature, viscoelastic effects. Moreover, while there are a great number of researches on the effects of the manifold angle, the channel height, and the die lip gap on the flow uniformity, there are a few researches on the effects of variation of the manifold cross section area. In terms of practical head design, some assumptions are extraordinary extreme cases. Accordingly, there is significant necessity of studying the effects of the cross sectional area of head channel. In this study, we firstly found viscosity model of PETG using a parallel-plate rheometer and a capillary rheometer. Secondly, a method combining the orthogonal array design and the numerical simulation was implemented to optimize the geometric parameters including the head channel cross sectional area with uniform flow behavior. The orthogonal array design was used to investigate the influences of the materials, geometric factors, processing condition. With computer simulation, head designers can repeat modifications to obtain the optimal head shape easily. Therefore we solved many cases of numerical method focusing on the variation of the head channel cross sectional area. Through all these process, the optimal values of the process condition and the geometric factors of the blow molding head were achieved.