It is crucial to guarantee the quality of aerospace products. Due to the existence of uncertainty sources(disturbance), there is a variation or excursion for the quality characteristic value from the target value. In order to evaluate the quality of a multidisciplinary optimization(MDO) design quantitatively, a quality variation mathematical model was proposed, which was used to quantitatively obtain the variation of every sub-discipline in a multidisciplinary from the target value and provide analysis and evaluation guidelines for MDO design. Simulation result of a NASA MDO example validated the effectiveness of the proposed mathematical model.