Abstract: High strength steel with yield strengths at 690 MPa has been widely used in primary structural members of steel structures because of their strength-to-self-weight ratios. Over the past two decades, there were a number of experimental investigations into mechanical properties as well as structural behaviour of high strength Q690-QT steel welded sections. It is evident now that, these welded sections will suffer from a significant reduction in their mechanical properties, i. e. both yield and tensile strengths as well as ductility, due to change in microstructures if welding is not properly controlled. These experimental investigations focus on describing the structural behaviour of welded sections, and the mechanism of the influence of different welding process parameters on the mechanical properties of high-strength steel welded connections has not been studied. Moreover, the improved welding technology that could avoid the strength reduction of welded high-strength steel could not be proposed. These studied are difficult to provide theoretical evidence for the practical engineering of high-strength steel materials in buildings and bridges. Therefore, a series of carefully planned and executed standard tensile tests were carried out to investigate and quantify effects of various line heat input energy onto the mechanical properties of the Q690-QT steel welded sections. A total of 12 standard tensile tests on cylindrical coupons of welded sections, 3 standard tensile tests on cylindrical coupons of base plate, 3 standard tensile tests on cylindrical coupons of welded metal were conducted, and full range deformation characteristics of these coupons were obtained through use of strain gauges and measurements on high resolution digital images. Both welding methods, namely, GMAW and SAW, were employed to prepare full penetration butt-welded section with various line heat input energy (1. 0, 1. 5, 2. 0, 5. 0 kJ/ mm). It should be noted that GMAW was performed with a robotic welding system while SAW was performed with an automated welding machine to attain high quality welding consistently. After comparison the tested data, the design suggestions for controlling the welded Q690-QT high-strength steel material strength in practical engineering will be proposed. The results show that almost all coupons of the welded sections tested in the present study, fracture occurred within the heat affected zones (HAZ) of the welded sections without any failure in neither the weld metal nor the base plates. For welded sections prepared with a line heat input energy equal to 1.0 or 1.5 kJ/ mm, there were almost no reduction in the mechanical properties of the welded sections. However, for these welded sections prepared with a line heat input energy equal to 2.0 kJ/ mm, only 90% of the yield and the tensile strengths of the base plates was attained. As for these welded sections prepared with a line heat input energy equal to 5.0 kJ/ mm, only 70% of the yield strength of the base plates was attained. Compared with the elongation tested data of base plates and welded sections, the ductility reduction of welded sections is more significant with the increase of heat input energy. Consequently, the effects of welding onto mechanical properties of the Q690 - QT steel welded sections have been successfully quantified, and the information is readily adopted in assessing their mechanical behavior according to various line heat input energy employed during welding. In practical engineering, the equal strength connection of high-strength steel materials could be achieved by setting the heat input energy as 1.0 or 1.5 kJ/mm.