The present invention relates to direct preparation of low-carbon olefins by using synthesis gas, and in particular to a catalyst and a method for directly converting synthesis gas into low-carbon olefins. In the method, synthesis gas is used as a reaction raw material and is subjected to a conversion reaction on a fixed bed or a moving bed. The catalyst is a composite catalyst, and is formed by compounding component I and component II in a mechanical mixing manner. The active ingredient of the component I is a metal oxide, and the component II is a molecular sieve having a CHA and AEI structure or one or more of metal modified CHA and/or AEI molecular sieves. The weight ratio between the active ingredient in the component I and component II is in the range of 0.1-20. The reaction process has a very high product yield and selectivity. The sum of selectivity of propylene and butylene reaches 40-75%, the sum of selectivity of low-carbon olefins including ethylene, propylene and butylene can reach 50-90%, and at the same time the selectivity of the byproduct methane is less below 15%, and thus the reaction process has good application prospects.