© 2019 The Authors Carbon fiber reinforced plastic (CFRP) is a prospective lightweight material in automobile industry. However, joining metal and CFRP is a great challenge. In the present study, an innovative process called coaxial one-side resistance spot welding (COS-RSW) is proposed to fabricate Al5052 (Al) and CFRP joints. Based upon our newly developed finite element code JWRIAN, the electric-thermal-mechanical coupled process of COS-RSW is modeled and validated with experiments. The influences of welding current, welding time and electrode force on CFRP molten zone are investigated in detail. The results show that the depth of molten zone has a strong correlation with both welding current and welding time. A relative low welding current or short welding time that results in the Al/CFRP interface temperature below the melting temperature of the resin matrix is insufficient to form a sound connection between the two sheets. While excessive current or too long welding time may lead to overheating of CFRP and decomposition of the resin matrix, which increases the risk of weak joining. Furthermore, it is found that the depth of molten zone increases even at the cooling stage, which indicates accurate simulation for both the heating and the cooling stages of the COS-RSW process is indispensable.