Two joint models with a scale of 1:3 were fabricated and fatigue tests were carried out to investigate the fatigue performance of steel truss web-concrete composite external joints and verify the reliability of the new structure applied to high-speed railway bridges. The fatigue damage mode, load-displacement relationship, and load-strain curve of the composite external joints were investigated, the fatigue force characteristics of the external joints under different stress amplitudes were investigated. The experimental results showed that the fatigue life of the composite external joint exceeds 2.51 million loading cycles under the design stress amplitude, the joint is not damaged, the stiffness is not reduced, and the fatigue resistance performance meets the requirements. Under the action of 1.4 times design stress amplitude, the measured fatigue life of the external joint is 1.4 million times. The fatigue damage mode of the external joint is the cracking of the exposed gusset plate on the tensile side, and the critical detail of composite external joint is the welding detail which connecting the exposed gusset plate and the stiffening plate. The fatigue crack causes the stress redistribution in the gusset plate, the strain in the gusset plate above and below the crack is reduced by 81% at most, and the strain in the gusset plate near the crack is increased by 33% at most. According to the S-N curve obtained by the one-point method, the fatigue life of the composite external joint corresponding to the design stress amplitude of 53.2MPa is 4.36 million times, which is 2.18 times of the design life.