Because quantification outcomes of primary performance parameter margins and uncertainties are inputs to a Quantification of Margins and Uncertainties(QMU) analysis，the method research of quantifying performance parameter margins and uncertainties plays an important role in QMU. In this paper, relative computing methods for figures of merit are presented according to different expressions of performance margin outcome. The changing trend of the key performance parameters over time for some component is determined by test data. The general method of quantifying performance parameter margins and uncertainties based on statistics theory is proposed. At last a relative example is introduced.