To study the cyclic softening/hardening behavior of high-speed railway gear steel 18CrNiMo7-6, RPL 100 electronic creep fatigue tester was used to carry out the uniaxial symmetrical strain cycling experiments on the gear steel under different strain amplitudes. Then, the ratchetting behaviour of gear steel 18CrNiMo7-6 under different stress conditions was tested to study the ratchetting evolution of gear steel under asymmetric stress cycling. The results show that the gear steel（18CrNiMo7-6） exhibits the cyclic softening characteristics under strain cycling and exhibits two stages of decaying ratcheting strain rate and constant ratcheting strain rate under asymmetric stress cycling. Then，a modified elasto-plastic cyclic constitutive model was established by introducing the evolution equation of ratchetting parameters related to the cumulative plastic strain into the Ohno-Abdel-Karim nonlinear kinematic hardening rule. The simulated results show that the model can reasonably predict the cyclic softening characteristics and the ratchetting evolution of gear steel（18CrNiMo7-6）.