Abstract: The fin-tube heat exchanger is common in the refrigeration industry. The expansion forming mechanism of the heat tube is of significant importance for refrigeration equipment, which determines its mechanical property and heat transfer performance. In this paper, a three-dimensional fluid-solid coupling model of the tube-fin heat exchanger is proposed. By using a unidirectional fluid-solid coupling transient method, the flow behaviors and deformation characteristics of the fluid and solid domains are numerically studied. Results show that the reasonable range of pneumatic expansion pressure is verified to be P = 12.5 MPa, which is consistent with the value derived from the theoretical equations. According to the variations of tube and fin stresses with time, the tube stresses at different fin-tube joints are greater than their yield limit of 66 MPa, and the fin stresses at different fin-tube joints are slightly greater than their yield limit of 132 MPa, which agree with the requirements of expansion forming process. After expansion, the average tube diameter increases with the pressure increases. The radial displacement of the heat exchanger tube is smaller in the horizontal direction and larger in the vertical direction, and the difference between maximum and minimum displacement is about 0.03 mm. The variation of the residual contact pressure with different expansion pressures was investigated, which exhibits three stages. When P < 11 MPa, the residual contact pressure increases with the expansion pressure. If 11 < P < 12.5, the residual contact pressure decreases with the increase of expansion pressure. While P > 12.5 MPa, the residual contact pressure stabilizes at 0.7 MPa. The numerical results indicate that when the expansion pressure makes the inner hole of the fin yield, increasing the expansion pressure will lead to incomplete expansion. Finally, the effect of holding time is studied, which show that changing the holding time has little effect on the expansion quality. The relevant results provide theoretical guidance for the actual engineering of the small fin-tube heat exchanger in the pneumatic expansion process.