The invention discloses a plate-fin heat exchanger for a gas heater, comprising a core body, seal heads, an inlet connection pipe and an outlet connection pipe, wherein the core body comprises a plurality of flue gas channels and sanitary water channels which are divided by partition plates in a rectangular cavity and are alternately connected; in the flue gas channel, flue gas side fins are arranged between two metal partition plates and are longitudinally arrayed, and the left and the right sides of each flue gas side fin are sealed by sealing tapes; in the sanitary water channel, water side fins are arranged between two metal partition plates and are transversely arrayed, the upper and the lower sides of each water side fin are sealed by sealing tapes, the left and the right ends of each water side fin are respectively communicated with the two seal heads, the seal heads at both sides are respectively connected with the inlet connection pipe and the outlet connection pipe of sanitary water, and the flue gas side fins and the water side fins are in continuous groove structures. The gas-water exchanger of the gas heater has high heat exchange efficiency, reduces the consumption of gas and achieves the purposes of saving energy, reducing device size and reducing production cost.