A leak-proof tube fitting assembly structure includes a liquid tube, a liquid tube fitting, and a nut for locking and connecting the liquid tube and the liquid tube fitting. The liquid tube fitting has a first connecting end, a first threaded connecting portion and a second connecting end that are axially arranged in sequence in a forward direction. The first threaded connecting portion has a first external thread. Thereby, the sealing performance of the connection of the liquid tube and the liquid tube fitting is improved to prevent leakage.