NOVELTY - Tetraethyl ammonium hydroxide, aluminum source, phosphoric acid, silicon source zeolite (SSZ-39) and water are mixed to obtain sol. The sol is aged, and hydrothermal synthesis is carried out to obtain silicoaluminophosphate-18 (SAPO-18) molecular sieve. Tubular porous support is immersed in aqueous solution of polymeric cationic compound to obtain aqueous suspension of seed crystal. The support is immersed in suspension of seed crystal, calcined and crystallized to form SAPO-18 film in outer wall of porous support. The support is calcined to obtain SAPO-18 molecular sieve film.    USE - Preparation of SAPO-18 molecular sieve film used for separating carbon dioxide-methane and carbon dioxide-nitrogen mixture.    ADVANTAGE - The method provides SAPO-18 molecular sieve film having excellent carbon dioxide selectivity, within a short period of time, using specific crystalline silicon source.    DETAILED DESCRIPTION - Tetraethyl ammonium hydroxide, aluminum source, phosphoric acid, silicon source zeolite (SSZ-39) and water are mixed to obtain sol. The obtained sol is aged for 4-24 hours, hydrothermal synthesis is carried out at 140-250 degrees C for 5-20 hours, centrifuged and washed to obtain SAPO-18 molecular sieve. Tubular porous support is polished, ultrasonic processing is carried out, and dried. The support is immersed in aqueous solution of polymeric cationic compound at constant temperature for 0.5-1 hour, and dried to obtain aqueous suspension of seed crystal. The pH of aqueous suspension of seed crystal is adjusted to 9-10 using alkali solution, and the support is immersed in aqueous suspension of seed crystal through the charge assembly to form uniform seed layer in the outer wall of the support. The resulting product is calcined at 350-500 degrees C for 4-6 hours to obtain seed crystal of support. The seed crystal of support is subjected to hydrothermal crystallization at 140-250 degrees C for 3-16 hours to form SAPO-18 film in the outer wall of porous support. The outer wall of porous support is washed, dried and calcined to obtain SAPO-18 molecular sieve film. The molar ratio of phosphorus pentoxide and aluminum oxide is 0.8-3. The molar ratio of silicon dioxide and aluminum oxide is 0.02-0.8. The molar ratio of tetraethyl ammonium hydroxide and aluminum oxide is 0.5-3. The molar ratio of water and aluminum oxide is 50-100.