Set Pair Analysis(SPA) is a new theory of quantitative analysis for certain and uncertain questions, and discrepancy degree coefficient “i” in connection degree formula is uncertain. In this article the discrepancy degree coefficient “i” is confirmed by Genetic Algorithm(GA),a set pair with “assessed standards” of water quality and “compared standards”is composed. The optimized connection degree formula was applied to assess the water quality of 12 stations in 8 rivers of Sichuan. The results of this assessment, which correspond with the real conditions, are basically in accord with those obtained by Bayes assessment models. In this way, the new effective approaches to the determination of discrepancy degree coefficient “i” and the assessment of water quality are suggested.
