The invention discloses a method for preparing cefalexin. The method comprises the steps of charging a D-2-phenylglycine ester derivative and 7-ADCA (amino desacetoxy cephalosporanic acid) in a molar ratio of (1.15-1.6):1, and adding a catalyst penicillin acylase with amount of 1-2 times of amount of the 7-ADCA for acylation reaction, wherein the coded gene sequence of the penicillin acylase is shown as SEQIDNO:3. The method effectively overcomes reverse reaction during enzyme condensation reaction, so as to greatly reduce the using amount of side chains, avoid the phenomenon that more impurities are generated when high side chains are consumed, and solve the problem that the objective product is difficult to purify; and besides, the 7-ADCA conversion rate can be greatly improved to be up to more than 98%, the product quality is further improved and the production cost is reduced.