The resolution of hydrogen embrittlement (HE) and hydrogen-assisted fatigue assessment in subsea pipelines is a critical technology for ensuring the large-scale development of offshore hydrogen industry. Hydrogen-assisted dislocation pinning near the crack tip is normally regarded as one of the key mechanisms of HE. However, there is a lack of a methodology to predict the hydrogen-assisted fatigue for pipeline steel influenced by dislocation pinning. Therefore, a piecewise linear elastic–plastic constitutive equation for pipeline steel in gaseous hydrogen environment is developed, in which the increases of yield strength due to both hydrogen-assisted dislocation pinning and the hardening of the pipeline after depinning are taken into account. To demonstrate the capability of the proposed model, it is introduced into a cyclic cohesive zone model (CCZM) for predicting hydrogen-assisted fatigue crack propagation of pipeline steel. The results shows that the proposed constitutive equation can well reflect the localization of plastic deformation and fatigue crack propagation acceleration caused by hydrogen-assisted HE.