In recent years, sub-synchronous oscillation incidents have been reported to happen globally, which seriously threatens the safe and stable operation of the power system. As the capacity of a single wind turbine generator is small, a wind farm with tens even hundreds of wind turbine generators makes the order of the model of the wind farm high with the wind turbine generators complex coupled with each other. Reasonable and effective equivalent modeling of the wind farm is the basis for the study on the reasons, characteristics and suppression of the oscillations in the wind power integrated power system. In this paper, the advantages and disadvantages of the existing relatively thorough methods for wind farm equivalent modeling is analyzed based on the recognition of the reasons for the sub-synchronous oscillations in the wind power integrated power system. Then the issues in the field of equivalent modeling of the wind farm that need to be further studied are summarized, hoping to provide references for the future study of small-signal dynamic equivalence of a wind power collecting system and sub-synchronous oscillations of the wind power integrated power system.