The invention provides a preparation method of total phenolic acid in a plant pithecellobium clypearia. The preparation method comprises the steps of crushing a pithecellobium clypearia raw material, extracting with pure water, condensing to form concrete, suspending with water, then putting on macroporous resin, eluting with acid water, removing water-soluble impurities, allowing phenolic acid substances to be maintained on the macroporous resin, eluting with a water-alcohol system to form a total phenolic acid extract which is also an effective part with pharmacological activity in the pithecellobium clypearia. The total phenolic acid in the pithecellobium clypearia has obvious antineoplastic activity, the pithecellobium clypearia, as traditional Chinese medicine, has lower toxic and side effects, and a new anti-tumor drug can be developed. The total phenolic acid in the pithecellobium clypearia can be prepared into various oral preparations. The method is reasonable in design and has the characteristics of simple technical process, low cost, low energy consumption and low pollution in a production process.