Accurate capacity evaluation in runway system at an airport is prerequisite for air traffic flow management and an essential content of airport planning. To deal with the problem that analytic logic is lack in MACAD (MANTEA Airfield Capacity and Delays) model in evaluating the capacity of two closespaced parallel runways, a new ultimate capacity model for two closespaced parallel runways is established by using a mathematical analysis method. The effect of departures in previous interarrival gaps in calculating the probability of inserting departures in current interarrival gap is specially analyzed and FAA airport capacity model's Qlogic is introduced so as to improve the accuracy of departure capacity evaluation result. The runway capacity envelope of an American commercial airport based on its reprehensive data is calculated respectively by using MACAD and this mode. The results show that the computation results of the model are basically consistent with those of MACAD model, but this model is more suitable for the definition of the runway capacity envelope.