The invention discloses a counter-flow heat exchange core body for fresh air ventilator, which is composed of a top plate and a bottom plate both having the same shape, and 2-500 heat exchange units, wherein the heat exchange unit is composed of a heat exchange membrane, an end seal and two lateral seals, the end seal is arranged at the upper surface of an end side of the heat exchange membrane, one end of the lateral seal is arranged on the upper surface of a side of the heat exchange membrane in a manner of aligning the other end side of the heat exchange membrane, the length of the lateralseal is 9/10 to 1/2 of that of the side of the heat exchange membrane, the heat exchange units which are vertically adjacent are 180-degree overlapped and are fixedly connected, the top plate is fixedly arranged above the uppermost heat exchange unit, and the bottom plate is fixedly arranged below the lowermost heat exchange unit. The counter-flow heat exchange core body according to the invention has the advantages of good heat transfer performance, high heat exchange efficiency, small volume, low resistance pressure drop and low cost for production.