Abstract: The Visul Environment software was used to establish the 3D model of the laser-arc hybrid welding joint of EQ70 steel. The temperature field of the laser-arc hybrid welding was numerically simulated by the SYSWELD software. The thermal cycle characteristics of the laser-arc hybrid welding and its effects on the microstructure evolution of the heat affected zone (HAZ) were analyzed, combined with the thermocouple thermometry and microstructure characterization. The results shown that the combined heat source with double ellipsoid and peak increasing cone could accurately simulate the temperature field of the laser-arc hybrid welding, and the arc zone, transition zone and laser zone had similar thermal cycles in the same HAZ micro-zones. The HAZ heating rate can reach to 400 ℃/s, the dwelling time above 1 100 ℃ was 0.79−1.33 s, and the t_8/5 was 4−6 s. The coarse grained HAZ (CGHAZ) and the fine grained HAZ (FGHAZ) were mainly comprised of lath martensite. The microstructure of the inter-critically HAZ (ICHAZ) was martensite and grain boundary carbides. The sub-critically HAZ (SCHAZ) composed of tempered martensite. The rapid heating and short high temperature dwelling time of the laser-arc hybrid welding limited the austenite grain growth to some extent, and the average grain size of the CGHAZ and FGHAZ were 42.7 μm and 19.8 μm, respectively.