Abstract: Objective The aim is to investigate the effect of slag entrapment defects on the weld damage behaviour of Q345R steel under tensile load. Methods Tensile experiments are carried out on welded specimen without weld defects and with slag entrapment defects, and combined with acoustic emission (AE) and infrared thermography monitoring technology to obtain real-time information on AE signals under tensile load and the surface temperature field of the damaged region. Results The results show that the slag entrapment defects lead to a decrease in the failure load of steel welded specimen, while the characteristic parameters such as acoustic emission signal amplitude, ring counts and energy are increased. In addition, the temperature at failure fracture of steel specimen containing slag inclusion defects is significantly lower than that of intact specimen. Conclusion The combination of AE and infrared thermography technology can effectively monitor the damage evolution process of steel welded specimen and provide a reference for steel structure health monitoring.