The submergence depth of the floating dock is adjusted according to the ballast water and determined by the quantity of the ballast water in the ballast tank. And the length of the air pipe in the ballast tank is the key factor affecting the quantity of the ballast water. The length of the air pipe in the floating dock is very important to the ship performance, because too long air pipe will affect the lifting capacity of the floating dock, while too short air pipe will cause the floating dock to capsize. It focuses on the theoretical calculation method of the length of the air pipe in order to solve the problem of accurately positioning the length of the air pipe in the floating dock, which has been proved by the sinkage test in a full scale. The results can provide reference for the subsequent design and manufacture of the floating dock.