This　paper　focuses　on　the　physical　essence　of　water＇s　quality　and,　based　on　exergy　theory,　makes　an　update　of　the　chemical　exergy　values　of　N　and　P　by　modifying　the　traditional　thermodynamic　model,　and　the　characterisation　factors　of　water　quality　are　calculated.　The　exergy　values　of　N　and　P　can　be　recalculated　as　156.15　and　1045.73　kJ　mol(-1),　respectively,　much　larger　than　traditional　results;　also,　the　exergy　values　of　phosphoric　acid　radical　and　nitric　acid　radical　become　positive.　The　results　are　suitable　for　use　in　life　cycle　assessment　or　other　water-related　research.