In this work, taking the fixed structure of rigid cylinder and elastic beam as the research object, the lock-in phenomena in vortex induced vibration of an elastic beam induced has been simulated based on the separated two way fluid solid interaction analysis method. The CFD method is used to calculate the flow field, and the elastic beam displacement is solved by the finite element method. The coupling is realized by transferring fluid force and structural displacement at the interface of fluid–solid domain, and the deformation updating of the euler mesh of the flow field is realized based on the dynamic mesh technology. According to the calculation results, lock-in phenomena occurs in the flow velocity range of 0.5–0.55 m/s, and the vortex shedding frequency remains unchanged and close to the second natural frequency of the elastic beam. At the velocity range, the vibration response of the free end of the elastic beam decreases with the increase of the flow velocity, which is in line with the structural resonance characteristics in the lock-in phenomena.