This　paper　investigates　the　interaction　of　water　with　multiple　circular　cylinders　subjected　to　earthquake　ground　motion　and　incident　linear　waves　respectively.　Accurate　and　efficient　finite　element　models　are　firstly　presented　to　calculate　the　earthquake-induced　hydrodynamic　forces　and　wave　forces　on　uniform　vertical　cylinders　with　arbitrary　cross-section.　An　elliptical　boundary　surrounding　all　the　cylinders　is　introduced　so　that　the　whole　domain　is　partitioned　into　unbounded　and　bounded　domains.　The　elliptical　exact　absorbing　boundary　condition　for　simulating　unbounded　domain　is　derived　in　the　elliptical　coordinate　system.　A　coupled　equation　is　obtained　by　adding　the　impedance　matrix　of　unbounded　domain　to　that　of　bounded　domain.　The　proposed　models　are　furtherly　used　to　investigate　the　earthquake-induced　hydrodynamic　forces　and　wave　forces　on　pile　group　with　two,　three　and　four　piles,　where　the　coefficients　of　pile　group　are　given.　The　results　indicate　that　interaction　of　water　with　multiple　cylinders　is　significant　when　the　relative　distance　between　two　adjacent　piles　is　less　than　three.　This　phenomenon　depends　on　the　relative　distance　between　piles,　width-depth　ratio,　wave　numbers,　and　the　direction　of　earthquake　or　incident　wave.　In　addition,　the　simplified　formulas　of　the　coefficients　of　pile　group　of　added　mass　are　proposed　for　engineering　application.