A double-layer medical fluid bag includes an inner wall, an outer wall, clean air filled between the inner and outer walls, and a piercing device installed between the inner and outer walls. When it is necessary to use a liquid medicine contained in the medical fluid bag, the piercing device is moved forward to pierce a plate to be pierced, so that the clean air can enter into the inner wall slowly to push the liquid medicine to flow downwardly to the outside. During the operation and use processes, the liquid medicine is completely isolated from outside air, so as to avoid the danger of having unknown outside air to enter into the medical fluid bag and endure the safety of using the medical fluid bag.
