In an array-type underwater apparatus for monitoring deformation of a reservoir landslide, an anchor is buried at an underwater monitoring point in a landslide mass, and a floating shell is configured to float on a water surface. A GPS sensor is configured to transmit and receive a GPS signal to obtain a real-time position of the floating shell, a water temperature sensor is used to obtain a water temperature-time relationship, and a gravity wave gauge is used to obtain a wave height-time relationship. An upper end of a pull cord is securely connected to the floating shell via a displacement compensation mechanism, and a lower end of the pull cord is securely connected to the anchor. The displacement compensation mechanism compensates for a displacement after the floating shell floats with a wave. An encoder-type displacement meter measures a real-time distance between the encoder-type displacement meter and the anchor.