The invention relates to method for identifying operating modal parameters of a linear time-varying structure based on memory-restricted principal component analysis. According to the method, the operating modal parameters of a dynamical system with time-varying structural characteristics can be identified by only using unsteady vibration response signals, the operating modal parameters of the structure are identified, and the dynamical variation characteristics of the system can be effectively monitored in real time, thus the method can be applied to vibration control, equipment fault diagnosis, health monitoring, and system structure analysis and optimization. The method is characterized in that the parameter characteristics of the system are identified by only using actual measured response signals, load signals aren't measured, and the method is proven according to mathematical theoretical analysis and experiment, so that a physical significance is endowed to the method. The invention further relates to an operating modal measuring device based on the method, and the device is used for measuring vibration response signals and identifying the operating modal parameters to obtain the real-time online dynamical characteristic change of the system structure, thus the device can be applied to fault diagnosis and health monitoring analysis of a large complex engineering structure (bridge, rail and the like).