A simulation model for a certain engineering vehicle was established to analyze the dynamic response of the operator in the accident of 90° lateral rollover. The motions of 50th percentile and 95th percentile human body model were simulated under these conditions: with and without wearing lap belt; with and without armrests. The restraint effectiveness of lap belt for different human sizes and the relationship between it and armrests were simulated by the established model. The results show that wearing a lap belt can reduce the likelihood of serious injury caused by operator ejection from the vehicle; the head and upper torso excursion for the larger operator model greatly increase the risk of interaction with hazardous features in the cab. When wearing a lap belt, the armrests tend to reduce the amount of head excursion and the severity of the head impact with the internal features of the cab, so the restraint effectiveness is better.