With advances in computer technology and digital human modeling methodology, it is possible to predict risks of potential injuries during manufacturing design prior to production, and make proactive ergonomic design for manufacturing assembly workstations involving human-machine interfaces. However, the productivity of manufacturing systems may be decreased, and this negative impact is difficult to evaluate in experimental and manufacturing environments. This research focuses on productivity issue for an automotive assembly workstation involving a lift assist device. To evaluate the productivity of the assembly workstation interfered by proactive ergonomic design, a prescriptive model of the automotive assembly system is developed and simulated with ProModel. This model and its simulation can not only evaluate the productivity, but also determine the maximum conveyor speed. Furthermore, this methodology using ProModel simulation to evaluate productivity and utilization described in this paper can be extended to evaluating other human-machine systems with dynamic, stochastic, and discrete-event characteristics.