Soup is usually pretty low in calories but can make you feel full.
So by eating just soup when losing weight you should be able to lose some weight with the soup only diet.
Cabbage soup is the best soup to help you lose weight because it's the lowest in calories but any other soup is also good for losing weight but you much change your lifestyle and get plenty of exercise as well while eating the soup and losing weight.
I lost 10 lbs in 2 months by sticking to the cabbage soup diet and then lost 10 lbs per month when I added exercise to my dieting.