The invention discloses a marinade soup, a medicated diet roast duck prepared by using the marinade soup, and a preparation method of the roast duck. The marinade soup contains the following raw materials: cloves, Chinese yam, Chinese angelica, cistanche, medlar, codonopsis pilosula, poria cocos, nutmeg, longan, dry ginger, shaved cinnamon bark, pepper, star anise, galangal, refined salt and cooking wine. The medicated diet roast duck prepared by using the marinade soup is prepared through the processes of adding water to the raw materials to prepare the marinade soup through stewing, preserving a cleaned duck in the marinade soup, applying honey to the duck and then roasting the duck. The preparation method comprises the following steps: 1) treatment of the duck body, i.e., stripping shanks from the cleaned duck and inserting an air injector into the skin to inject air; 2) stewing of the marinade soup; 3) preservation, i.e., preserving the duck body in the marinade soup for 3h, then fishing out the duck body and draining off water; 4) steaming, i.e., steaming the duck body for 1h with stir fire, and then applying honey after taking out the duck body; and 5) roasting, i.e., roasting the duck body for 40min with stir fire, thus obtaining the medicated diet roast duck. The medicated diet roast duck prepared by using the marinade soup is rich in nutrition and mellow in mouthfeel and has the efficacies of essence enrichment, kidney invigoration, spleen tonifying, appetizing, and invigoration of qi and blood.