The invention provides a length-adjustable steel structure beam which comprises a beam body frame, the beam body frame is a groove body with three sealed surfaces and one open surface and comprises a first clamping groove formed in a top plate, second clamping grooves formed in the middles of the inner side surfaces of two side plates and grooves formed in the bottom ends of the side plates, a first connecting plate is arranged between the first clamping grooves of the two adjacent beam body frames, a transversely-arranged I-shaped second connecting plate is arranged between the second clamping grooves, and a third connecting plate is arranged between the grooves in the bottom ends of the two side plates. The length-adjustable steel structure beam provided by the invention has the beneficial effects that three connecting plates play positioning and limiting roles, so that the welding precision can be ensured; the configuration and the attractive appearance of the combined steel structure beam are ensured; meanwhile, the use is very convenient, the large-scale production is also facilitated, and the overall construction cost is reduced. The strength of the steel structure beam is also improved, so that the safety performance is improved.