NOVELTY - Preparing composite film for separating mixed gas of carbon dioxide and firedamp comprises: (a) pre-treating a porous carrier; (b) dispersing the Silicoaluminophosphate (SAPO)-34 molecular sieve into organic precursor solution; (c) coating the film preparing liquid on the surface of the porous carrier; (d) carbonizing the SAPO-34/organic matter composite film; and (e) applying SAPO-34/C composite film in separating mixture of carbon dioxide (CO2)/methane (CH4) or H2/CH4, where the SAPO-34/C composite film represents high selectivity and high flux.    USE - The method is useful for preparing composite film for separating mixed gas of carbon dioxide and firedamp (claimed).    ADVANTAGE - The method solves the problem in preparing the existing carbon molecular sieve film that it is difficult to improve the gas flux while maintaining the selectivity. This simple preparation method is used for effectively preparing high selectivity and high flux films.    DETAILED DESCRIPTION - Preparation composite film for separating mixed gas of carbon dioxide and firedamp, where SAPO-34 molecular sieve with strong capability for adsorbing CO2 is mixed with carbon molecular film; in this case, the composite film is accelerated to infiltrate CO2 selectively; the SAPO-34/C composite film has high selectivity and high flux to CO2/CH4 comprises: (a) pre-treating a porous carrier; (b) evenly dispersing the SAPO-34 molecular sieve into organic precursor solution for preparing carbon molecular sieve; in this case, even film preparing mixed liquid containing SAPO-34 molecular sieve is obtained; (c) evenly coating the film preparing liquid on the surface of the porous carrier through all kinds of coating methods until the carrier is completely covered and then drying the carrier; in this case, a SAPO-34/organic matter composite film is obtained; (d) carbonizing the SAPO-34/organic matter composite film into a SAPO-34/C composite film at high temperature or in vacuum under oxygen-free inert gas; and (e) of applying the SAPO-34/C composite film in separating mixture of CO2/CH4 or H2/CH4, where the SAPO-34/C composite film represents high selectivity and high flux.