Lili, google the following information for you. I think potato juice is fine. Just may not taste good. Anyway, I won't try.
I would rather mix apple, lime (or lemon), and carrot.
Potato
An excellent source of vitamin C and high in carbohydrates, potassium, calcium and iron. The most nutritious part of the potato is the skin. During World War I, a group of German soldiers, near starvation, were forced to live on raw potatoes. The soldiers with gastric disorders were relieved and cured. After the war, Dr. J. F. Magerl began treating gastric patients with raw potato juice. After 10 days of the treatment, most of the patients showed no symptoms.
Potato juice is a wonderful addition to any vegetable juice. When potatoes are juiced, a white silky paste will form on the bottom of the glass. This is potato starch and is mucus-forming with little nutritional value. Of all the vegetables, potatoes are slightly mucus-forming, but are still a valuable food, especially when juiced.
Look for potatoes with eyes, revealing the fact that they are alive with enzymes, ready to sprout, given the opportunity. They should be firm, without black spots and with no green color, which can cause a slightly toxic effect. Never juice a green potato. Will keep well in a cool, dry place for a number of months.