The invention relates to a catalyst containing a heteroatom-doped molecular sieve and a method for preparing low-carbon olefin by directly converting synthesis gas. The catalyst is a composite catalyst; a component I and a component II are compounded together in a mechanical mixing manner; wherein the active ingredient of the component I is a metal oxide; the component II is the heteroatom-doped molecular sieve; the molecular sieve has a topological structure of CHA or AEI, skeleton atoms comprise Al, P, O and Si, and heteroatoms can be one or more than two of ions from divalent Mg, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Zr, Mo, Cd, Ba, Ce, trivalent Ti, Ga and tetravalent Ge; the weight ratio of the active ingredient of the component I to the component II is in a range of 0.1-20 times. The reaction process has very high low-carbon olefin selectivity, the sum of the selectivity of low-carbon olefins including ethylene, propylene and butylene can reach 50-90%, meanwhile, the selectivity ofbyproduct methane is lower than 7%, and meanwhile, the method has a very good application prospect.