The invention aims to provide a gas sensor and a preparation method thereof. The preparation procedures of sensor electrode depiction and a sensitive material coating process are concise and efficient, and a sensitive material has good consistency. Meanwhile, the service life is longer than that of an existing product. In order to achieve the purposes, the preparation method comprises the steps of1, electrode depiction; and 2, material deposition, wherein ultrafast lasers directly focus on the surface of a sensitive material tablet through an insulated substrate, the gas sensitive material forms a smoke beam which deposits upwards and is attached to the insulated substrate to form a flocculent grid three-dimensional structure in the scanning process, and preparation of the gas sensor is completed after scanning is over. By the adoption of the technical scheme, the two steps of electrode depiction and material deposition can be completed just through one laser generator in the whole process; the whole processing process is at relatively low temperature and belongs to cold processing; and the gas sensor can be directly put into use without a cooling step after being processed.