In this study, we investigated the post-welding behaviour of S690Q high strength steel butt joints experimentally. Three S690Q high strength steel butt joints were welded with 8 mm thickness plate by shield metal arc welding with different welding heat inputs. Microstructure observation and micro-hardness test of heat-affected zone were employed to reveal the welding influence on the joints at the micro level, and tensile test of S690Q high strength steel butt joints showed the welding impact at the macro level. The microstructure test indicated that the main microstructure of the S690Q high strength steel was tempered martensite and was transformed to granular bainite in the coarse-grained heat-affected zone and to ferrite and cementite in the fine-grained heat-affected zone after welding of the joints. In the tempering zone, some of the tempered martensite decomposed to ferrite. Based on the hardness test results, it can be observed there is a soft layer in the heat-affected zone with lower hardness compared with the base material. In addition, the width of the heat-affected zone increased in the joint welded with higher heat input. The subsequent tensile test showed that the formation of a soft layer directly impairs the tensile behaviour of S690Q high strength steel butt joints, and the strength deterioration becomes more serious with the increase of the welding heat input.