The　effect　of　water　compressibility　on　the　seismic　responses　of　arch　dams　is　not　well　understood.　In　this　paper,　a　numerical　model　is　developed　with　rigorous　representation　of　the　dynamic　interaction　between　arch　dam-waterrock　foundation.　The　model　is　applied　to　the　seismic　response　analysis　of　an　arch　dam　with　a　height　of　292m　designed　to　a　seismic　intensity　of　IX.　It　is　shown　that　consideration　of　the　water　compressibility　clearly　decreases　the　stress　responses　at　key　positions　of　the　dam,　while　the　added　mass　model　gives　a　conservative　estimate.