NOVELTY - Preparation of gas sensor comprises adding deionized water into malysite and pink salt, uniformly stirring, dropping ammonia, controlling pH value, centrifugally separating obtained deposits, rinsing and drying; grinding and calcining, putting tin oxide-ferric oxide (SnO2-Fe2O3) nanocomposite powders into mortar, and gradually dropping chloroauric acid and ruthenium chloride noble metal salt mixed solution; uniformly stirring, immersing, drying and cooling; and grinding gas sensor material, forming into paste, uniformly coating on surface of alumina ceramic tube and annealing.    USE - Method for preparation of gas sensor for detecting refrigerating fluid HC-600a(RTM: 2-methylpropane) (claimed) and isobutane used in metal oxide semiconductor preparation.    ADVANTAGE - The gas sensor is sensitive to HC-600a(RTM: 2-methylpropane) refrigerating gas, and has good selectivity on interference gas, short response resuming time and good stability.    DETAILED DESCRIPTION - Preparation of gas sensor comprises adding deionized water into malysite and pink salt, where ionic ratio of tin and iron is at 1-15:100; uniformly stirring to prepare mixed solution, dropping ammonia, controlling pH value at 5-9, centrifugally separating obtained deposits, rinsing and drying to obtain rufous granules; grinding, and calcining for 2 hours at 650 degrees C to obtain SnO2-Fe2O3 nanocomposite powders; putting SnO2-Fe2O3 nanocomposite powders into mortar, and gradually dropping chloroauric acid and ruthenium chloride noble metal salt mixed solution at mole ratio of 1:1; controlling mass ratio of noble metal element to SnO2-Fe2O3 at 0.1-1.0:1000, uniformly stirring using glass rod, immersing for 30 minutes, drying for 1 hour at 100 degrees C, and cooling to prepare noble metal doped SnO2-Fe2O3 mixed gas sensor material; and grinding gas sensor material, forming into paste with deionized water and polyethylene glycol, uniformly coating on surface of alumina ceramic tube and annealing at 500 degrees C for 2 hours to obtain the product. The malysite comprises ferric chloride, iron sulfate or iron nitrate. The pink salt comprises stannic chloride pentahydrate.