Abstract: Objective To clarify temperature evolution and material flow behavior of friction stir lap welding process was helpful to analyze influence of friction stir multiple physical fields on weld quality. Methods A numerical model with quasi-static, high reliability and high precision accuracy, was established for simulation of friction stir lap welded dissimilar alloys of aluminum and steel based on coupled Eulerian-Lagrangian (CEL) model in ABAQUS environment. Evolution of temperature field, equivalent plastic strain field and stress field in friction stir lap welded joints of 6061 aluminum alloy (top) and Q235 steel (below) were studied. Results The results indicated that probe length had significant impacts on temperature and deformation of lap welded joints. In welding process, temperature and strain of welded joints in longitudinal section decreased gradually from top to bottom and from center to both sides. Long probe could increase heat production and material deformation, which promoted material flow process. At this time, internal stress of material would also increase. Moreover, probe length had little effect on stress distribution in aluminum alloy plates. After material mixture, EVF cloud map indicated that long probe facilitated material mixture at the interface, and materials mixed more thoroughly in the retreating side, which accelerated formation of hook like structure. Conclusion Under the action of long probe, with the probe inserted into steel, steel and aluminum engaged mechanically, significant difference in mechanical performance made it easy to form defects in the advancing side and the below position of probe. Therefore, as an important factor affecting welding quality, probe length should be selected before conducting friction stir welding tests on dissimilar materials.