In this study, the post-welding behaviours of S690Q high strength steel (HSS) butt joints are investigated experimentally. Three S690Q HSS butt joints are welded with 8mm thickness plate by shield metal arc welding (SMAW) with different welding heat inputs. Microstructure observation and micro-hardness test of heat-affected zone (HAZ) are employed to reveal the welding influence on joints in micro level, and tensile test of S690Q HSS butt joints shows the welding impact in macro level. The microstructure test indicates that the main microstructure of S690Q HSS is tempered martensite and is transformed to granular bainite in the coarse-grained HAZ (CGHAZ) and to ferrite and cementite in the fine-grained HAZ (FGHAZ) after welding of joints. In tempering zone, some of the tempered martensite decomposes to ferrite. Based on the hardness test results, it can be observed there is a soft layer in HAZ with lower hardness compared with base material. AdditionallyIn addition, the HAZ width is increased in the joint welded with higher heat input. The subsequent tensile test reveals shown that the deterioration effect of welding on the strength of S690Q HSS butt joints was due to the formation of “soft layer” during welding. All joints failed in HAZ, and the strength deterioration becaomes more serious with increase of the welding heat input.