A Portulaca oleracea L. extract feed additive and a preparation method thereof are provided. The Portulaca oleracea L. extract feed additive takes a Portulaca oleracea L. extract and a carrier as raw materials. The Portulaca oleracea L. extract is prepared by the ultrasonic auxiliary water extraction. The preparation method of the present invention has less raw material waste, high activity component content in the extract and acquisition rate of the extractum, which facilitates improving the effect efficiency and reducing production cost. The Portulaca oleracea L. extract feed additive as a natural plant feed additive is capable of improving the animal's metabolism, increasing the animal's relative weight gain, improving the animal's survival rate, reducing the disease incidence of various diarrhea symptoms, decreasing the animal's stress response and increasing the animal productivity.