For the modern industrial technology, people are trying to find a optimal design to reduce cost and time. Ergonomics is an essential issue to consider in design. To make sure the comfort of passenger car is preferable, it must design optimal passenger sitting posture in the concept design period. This essay provided a method about optimal design in layout passenger sitting posture. It builds a virtual prototype which includes manikin and auto body in RAMSIS to simulate actual passenger sitting posture, and then it analyzes the comfort. Based on these analyses, it chooses back angle of passenger and seat height as variable and the overall discomfort as target value. The experiment was been designed according the method of full factorial design. So the experiment includes 36 trials with two factors and six levels of each factor. And then the object function is estimated by using response surface methodology (RSM). The significance analysis also was been studied. The analysis showed that the model is significant in the significance level of 1%. The optimal passenger sitting posture was got from the object function in Matlab. The discomfort of passenger has decreased 13.3%. The passenger comfort has been improved remarkably.