Abstract: The effect of the non-metallic inclusion in 14MnNbq steel on the mechanical properties of welded joints is discussed and analyzed. The result shows that there are a lot of sulfide inclusions (MnS, FeS) in 14MnNbq steel. These non-metallic inclusions badly dispersed in the steel are harmful to the mechanical properties of the welded joint. Their existence will decrease the impact value of the welded joint at low temperature, and increase the discreteness of the measured data.