The present disclosure discloses a flexible display auxiliary device and a foldable mobile terminal. The flexible display auxiliary device includes a platform, a first stage, a second stage, a sliding mechanism and an enabling mechanism. The first stage is used for fixing the first end of the flexible display screen, the second stage is used for fixing the second end of the flexible display screen opposite to the first end. The second stage is fixedly arranged on the sliding mechanism, the sliding mechanism is arranged on the platform, the enabling mechanism is connected to the sliding mechanism to control the sliding mechanism to move on the platform so as to change the relative distance between the first stage and the second stage and change the tension on the flexible display screen. In the above manner, the present disclosure enables the flexible display to always be in a flat state.