The effects of the second pulse in resistance spot welding (RSW) on the mechanical properties of dual phase (DP600) steel were investigated. The experimental results show that the addition of the second pulse can give rise to the formation of acicular ferrite, retained austenite and chromium carbides, which can markedly improve the cross-tensile strength (CTS) comparing to that treated by the single pulse technology. Meanwhile, the formation mechanism of acicular ferrite, bearing the cubic to cubic orientation relationship with respect to the surrounding martensite lath, was revealed by the analysis of the thermal history of DP600 steel under different RSW processes. The effect of microstructure on the fracture behavior was also discussed.