The invention discloses an active carbon catalyst, a preparation method thereof and application thereof in preparing acrylic acid by dehydrating lactic acid. The catalyst is obtained by taking active carbon (AC) or acidified and oxidized active carbon (HOAC) as the primary active component, introducing one or more than one of acid phosphate or metaphosphate or pyrophosphate with immersion method or direct grinding mixing method and drying. The catalyst is used for preparing acrylic acid by dehydrating lactic acid with high lactic acid conversion rate and acrylic acid selectivity, wherein the lactic acid conversion rate is 100% and the acrylic acid selectivity is up to 65%.