Groundwater　is　an　important　source　of　water　in　Beijing.　Hydrochemical　composition　and　water　quality　are　the　key　factors　to　determine　the　availability　of　groundwater.　Therefore,　an　improved　integrated　weight　water　quality　index　approach　(IWQI)　combining　the　entropy　weight　method　and　the　stochastic　simulation　method　is　proposed.　Through　systematic　investigation　of　groundwater　chemical　composition　in　different　periods,　using　a　hydrogeochemical　diagram,　multivariate　statistics　and　spatial　interpolation　analysis,　the　spatial　evolution　characteristics　and　genetic　mechanism　of　groundwater　chemistry　are　discussed.　The　results　show　that　the　groundwater　in　the　study　area　is　weakly　alkaline　and　low　mineralized　water.　The　south　part　of　the　study　area　showed　higher　concentrations　of　total　dissolved　solids,　total　hardness　and　NO3--N　in　the　dry　season　and　wet　season,　and　the　main　hydrochemical　types　are　HCO3--Ca　and　HCO3--Ca-Mg.　The　natural　source　mechanism　of　the　groundwater　chemical　components　in　Chaoyang　District　includes　rock　weathering,　dissolution　and　cation　exchange,　while　the　human-made　sources　are　mainly　residents　and　industrial　activities.　Improved　IWQI　evaluation　results　indicate　that　water　quality　decreases　from　southwest　to　northeast　along　groundwater　flow　path.　The　water　quality　index　(WQI)　method　cannot　reflect　the　trend　of　groundwater.　Sensitivity　analysis　indicated　that　the　improved　IWQI　method　could　describe　the　overall　water　quality　reliably,　accurately　and　stably.