Coherency-based dynamic equivalence is a commonly used method to establish the equivalent model of a wind farm, in which it is considered that a group of wind turbine generators (WTGs) of the same type and similar operating states could exhibit coherent dynamic performances to the external power system and its total dynamic output characteristics can be reflected via a single WTG. In this paper, theoretical basis and tenable conditions for this dynamic equivalent method is mainly studied based on small-signal model. Therefore, full-order linearized state-space model of a group of WTGs (GW) is established at first. Secondly, drawing on modal decomposition in modal analysis, the equivalent decomposition of a GW dynamic system is derived. Based on the results of the derivation, in analogy with mode shape, controllability and observability, reasons and tenable conditions which WTGs in a GW could exhibit coherent dynamics to the external AC grid and the complete dynamic performance of the GW can be reflected by a single WTGs are discussed. Finally, the proposed methods and conclusions are validated through study cases. The methods and conclusions provide theoretical basis for coherency-based dynamic equivalent of wind farm (CDEWF) and are of certain significance to be guidance or reference in researches on WTGs clustering in CDEWF.