Abstract: Objective Effect of double-pass friction stir welding (FSW) on microstructure and mechanical properties of lap joint were studied. Methods Dissimilar materials lap joints between 5083 aluminum alloy and FH36 steel with thickness of 6 mm were obtained by double-pass friction stir welding under the same plunging depth of welding tool with different rotating speed and traversing speed. Influence of the double-pass welding process on the forming of welded joints was discussed through microstructure characterization and mechanical properties tests. Effect of welding parameters on the distribution of intermetallic compounds (IMCs) in welded joints and strength of welded joints were described. Results The results showed that compared with single-pass FSW, double-pass FSW method could reduce or eliminate tunnel defects in welded joints. There was an obvious IMCs layer at the Al/steel interface. With the increase of rotating speed, thickness of IMCs layer increased, and the maximum tensile shear load increased first and then decreased. With the increase of traversing speed, thickness of IMCs layer decreased, whose distribution became more uneven, and the maximum tensile shear load decreased gradually. Conclusion Compared with single-pass friction stir welding, tensile shear properties of welded joints were improved by double-pass friction stir welding. When rotating speed was 800 r/min and traversing speed was 30 mm/min, the tensile shear load of welded joints reached the maximum value of 8.7 kN, which was 93% higher than that of single-pass FSW with the same welding parameters.