The driver’s seat in heavy trucks is designed for an upright driving posture with narrow back and cushion angles; thus, the seatback offers very little support. This makes the sitting posture prone to shifting during long trips, leading to loss of comfort and increase in fatigue. Sitting posture stability allows initial posture to be maintained during long drives, and the lack of stability causes fatigue and body pain during the drive.This study confirmed that enhancement of sitting posture stability of the driver’s seat in heavy trucks requires appropriate support from the cushion. The study also analyzed the support characteristics of each part of the cushion, and presented development guidelines of new cushion. Although subjective assessments of sitting posture stability have been performed, this study presented a method for quantitative and efficient assessment of sitting posture stability using the PAM-COMFORT simulation tool and virtual testing. In particular, the interaction between the human body and the seat was analyzed according to different seat design variables using an FE human model and an Hip point machine (HPM-II, J4002). Moreover, virtual tests, such as LLD (Local Load Deflection) and Penetration (physical deformation of the seat), were used to compare and test the support characteristics of seats. Based on the results, a case study was conducted prior to prototyping to develop an optimal plan for seat support characteristics that can help improve sitting posture stability. Subsequently, the prototype was built according to the optimal plan and the results of real-world and virtual testing were compared to verify the reliability of the virtual test.