The invention discloses concentrated wine containing purple onions, brandy with an onion flavor and a preparation method of the concentrated wine and the brandy. Grapes and the purple onions are taken as raw materials, and onion pieces and a mixture of grape juice, grape skin and seed are subjected to biological fermentation. The fermentation process is performed as follows: yeast aerobic fermentation and alcohol fermentation are performed, and the concentrated wine containing the purple onions and a byproduct, the brandy with the onion flavor, are prepared after freeze concentration, reduced pressure distillation, ultra-filtration sterilization, sterile filling and other processes. With the adoption of the preparation method, the problems about onion odor elimination and effective protection of non-volatile flavor substances and nutritional ingredients are solved, the concentration of nutritional and functional materials of the purple-onion-containing concentrated wine prepared with the method is increased by 5-6 times, the onion flavor and the fragrance of the wine and the brandy are rich, the tastes are extremely good, and the health-care function is remarkable. Little residual onion odor in the byproduct, the brandy with the onion flavor, can generate a special stimulation to the taste and the smell of a drinker, and accordingly, the unique flavor and taste of the brandy with the onion flavor are formed.