The invention relates to an earth rock cofferdam water seepage treatment structure which includes a closure dike. A sub-cofferdam is arranged at the position of a downstream slope foot of the closuredike, underwater concrete is poured on the position, between the sub-cofferdam and the closure dike, of a river bed, and a clay core wall is constructed above the underwater concrete in a filling manner. A construction method of the earth rock cofferdam water seepage treatment structure includes the following steps that 1) dike filling is carried out, wherein a one-way single dike vertical plugging method is adopted for dike filling; 2) cofferdam tightness is carried out, wherein the sub-cofferdam is built at the position of the downstream slope foot of the closure dike, and the underwater concrete is poured on the position, between the sub-cofferdam and the closure dike, of the river bed; 3) drainage pipe laying is carried out, wherein a plurality of drainage pipes in the direction of theriver bed are paved above the underwater concrete, and seepage water in a dike body is drained; and 4) clay core wall filling is carried out, wherein the clay core wall is constructed above the underwater concrete in the filling manner, and inverted filter layers are constructed on the two sides of the upstream and the downstream of the clay core wall in a filling manner. By adopting the structure and the construction method, after air tightness of the cofferdam on the river bed is carried out, the anti-seepage effect is improved, the water seepage of the dike body is effectively reduced, andthe regular drainage cost is reduced.