Abstract: The inherent rotary inertia of the new power system with renewable energy as its main body will be significantly reduced mostly due to the large-scale reduction of the thermal power, and the distribution of the equivalent inertia will also be changed due to the large-scale investment of new rapid power control measures. Facing this new system environment, it is urgently needed to improve the applicability of the inertia evaluation models and methods. Therefore, an evaluation method of the equivalent inertia time coefficient (EITC) for the power system based on the frequency trajectory is proposed. Firstly, the main factors affecting the space-time distribution characteristics of the EITC are qualitatively analyzed. Secondly, by extending the first-order model of the inertial system dynamic equation, the active power changes after the comprehensive actions of the multiple control measures are equivalently treated as a time-varying EITC in the first-order model. On this basis, the quantitative relationship between the EITC and the frequency trajectory is established, and the instantaneous gain and the time-window average indexes of EITC are also defined. Furthermore, referring to the key information provided by the standards of the Under-frequency Load Shedding and the Rate of Change of Frequency (RoCoF), and based on a special single AC synchronous machine system, its reference inertia time constant and the corresponding reference transient frequency drop trajectory are extracted. Taking the reference transient frequency drop trajectory as a unified ruler, the dynamic assessment of the EITC's distributions and the identification of the weak points can be achieved by only collecting the frequency trajectory information at different electrical positions. Simulations show that the space-time distribution characteristics of the EITC in the new power system with comprehensive actions of the multiple power sources and the inertia/frequency control measures can be evaluated by the proposed method. This method also provides technical support for the further researches on the optimization of the grid layout and the operation strategies of various power sources and control measures.