The planar and compact type all-solid state electrochemical gas sensing device based on yttria-stabilized zirconia (YSZ) and Zinc molybdate (ZnMoO₄) composite sensing electrode (SE) was proposed and fabricated to effectively detect ppb-level H2S at high temperature. The gas sensing property of different SEs (ZnMoO₄, CoMoO₄and NiMoO₄) is compared and explored. The sensor attached with ZnMoO₄-SE processes the highest response value of - 62.5 mV to 1 ppm H₂S and lower detection limit of 5 ppb H₂S at 500℃. The response and recovery times of the sensor to 500 ppb H₂S are 20 s and 58 s, respectively. The sensor also displays high sensitivity, good continuous reproducibility, selectivity and stability of humidity and high temperature at 500℃. In addition, the sensing principle involving mixed potential model was proposed and verified using polarization curves.