Steel structure welding damage is a result of a large number of instantaneous heat input in the process of welding. Welding produces some welding defects including residual stress cracks, porosity, inclusions and bite edge. Weld defects bring adverse effect to the steel structure including overall mechanical properties, fatigue strength and stability, especially the seismic load and impact load under dynamic load effect, which reduces the security of the steel structure. The paper summarizes the possible welding steel structure in the process of welding defects, specifically for the welding problem of unsaturated by components in the different welding rate of dynamic characteristic test to study the dynamic characteristics of the unsaturated welding of steel structures. To analyze the impact of different support on the test results, it analyzes five different welding length of steel beams on the modal analysis with welding rates of 17.3%, 33.3%, 40%, 33.3%, 70%, 100%, collects modal data and compares different welding rates of steel structures.The influence of different welding rates on the dynamic charateristics of steel structures is analysed as the conclusion, which provides theoretical and experimental basis for the design and construction of steel structures.