Basedontheone-dimensionalsalinitytransportequationwithconstantdiffusioncoefficient,andseparatedwaterflowvelocityintorunoffandtidalcurrentwiththesingle-frequencyinanidealizedestuary,thesimplestunsteadyanalyticalsolutionofsalinityintrusionisdeducedandtheestimationformulaofdiffusioncoefficientisobtainedinthispaper.Theunsteadysolutionindicatesthatsalinityprocessinestuariesresultsfromtheinteractionofrunoffandtidalcurrent,anditsamplitudeisindirectproportiontotheproductofthevelocityofrunoffwaterandtheamplitudeoftidalflowvelocityandininverseproportiontothediffusioncoefficientandthetidalangularfrequency,anditsphaselagtidalflowwithπ/2whichrevealsthebasicfeaturesofthemaximumsalinityappearingafterfloodslackandtheminimumsalinityappearingbeforeebbslackundertheeffectofrunoff(theadvanceorlagtimeisrelativetothemagnitudeofrunoffandtidalflow).Accordingtothemeasuredflowvelocityandsalinitydata,thesalinitydiffusioncoefficientcouldbeestimated.Finally,withthefielddataofobservingsitesonthedeepwaternavigationchanneloftheYangtzeEstuary,thediffusioncoefficientiscalculatedandacomparativeanalysisofsimulatedandmeasuredofsalinityprocessismade.Theresultsshowthatthesolutioncancomprehensivelyreflectsthebasiccharacteristicsandprocessesofsalinityintrusionundertheinteractionofrunoffandtidalflowinestuaries.Thesolutionisnotonlysuitablefortheoreticalresearch,butalsoconvenientforestimatingreasonablephysicalparametersandgivingtheinitialconditioninthesalinityintrusionnumericalsimulation.
