Trickle bed reactors are the most common gas-liquid-solid three-phase reactors involving a solid catalyst, and gaseous and liquid reactants. If solvents in the liquid phase are indissoluble in each other, gas-liquid-liquid-solid four phases exist in the reactor. Deprotection of amino acid was studied in a trickle bed reactor as a typical example of a four-phase system. A reactant (carbobenzoxy phenylalanine) could be dissolved in an organic phase and hydrogenated to a product (phenylalanine) using a Pd/C solid catalyst. Phenylalanine had hydrophilic properties and transferred from the organic phase to the aqueous phase. When two liquids were continuously introduced into the four-phase reactor, the product (phenylalanine) was deposited on the surface of the catalyst and the catalytic activity gradually decreased. Periodic operation was adopted to solve this problem. Only distilled water was periodically introduced into the reactor to wash the catalyst, which could be completely regenerated after this washing period. The optimal reaction period was determined by simulation to obtain a high concentration of the product.