A catalyst containing LF-type B acid preparing ethylene using direct conversion of syngas is a composite catalyst and formed by compounding component A and component B in a mechanical mixing mode. The active ingredient of the component A is a metal oxide; the component B is a zeolite of MOR topology; and a weight ratio of the active ingredients in the component A to the component B is 0.1-20. The reaction process has an extremely high product yield and selectivity, with the selectivity for light olefin reaching 80-90%, wherein ethylene has high space time yield and can reach selectivity of 75-80%. Meanwhile, the selectivity for a methane side product is extremely low (