The invention relates to a horizontal displacement monitoring device and a horizontal displacement monitoring method in an overall horizontal-displacement construction and operation process of a concrete dam. The horizontal displacement monitoring device comprises a horizontal displacement monitor, a pipe sleeve, a datum line and a floating device; the pipe sleeve is a segmented extensible pipe sleeve; the datum line is arranged perpendicular to the horizontal plane; the pipe sleeve is arranged outside the datum line in a sleeving manner so that the datum line has an enough relative movement space relative to the sleeve pipe; the floating device comprises a floating cylinder protective hood, an oil tank and a floater, wherein the oil tank and the floater are arranged inside the floating cylinder protective hood; antifreeze non-volatile low-viscosity liquid is arranged inside the oil tank; the floater floats on the liquid under the effect of the buoyancy force of the liquid and is not limited by other components; the pipe sleeve and the floating device both synchronously rise with rising of a dam body in a construction process of the dam; and the horizontal displacement monitor is fixed to a measuring point position of the dam body and is used for monitoring changes of relative horizontal displacement of the dam body and the datum line at an instrument installation position. Thehorizontal displacement monitoring device provided by the invention does not affect the construction of the concrete dam and can realize continuous monitoring on the horizontal displacement of the dam in the overall processes of the construction period and the operation period of the dam.