Based on the offset friction stir welding (FSW) characteristics of the Al-Cu dissimilar, the ANSYS software was used to simulate the transient temperature field and the welding thermal cycle curve of the fusion zone during the welding process. The maximum temperature in the transverse, longitudinal and thickness directions of the weld during the welding stabilization stage were compared and analyzed. The effect of the welding parameters on the welding temperature was investigated. The results showed that the rotation speed played a leading role in the welding temperature field. The computed results were in good agreement with the actual thermal cycling curve of feature points, which verified the accuracy of the heat source model. The results of the simulated temperature field indicated that the peak temperature appeared in the position of the partial aluminum alloy side of the weld center.