This paper presents the algorithm and control logic used in SCS-500L power oscillation islanding control device. Based on the line power oscillation trajectory measurements， this method has several distinct characteristics such as clear principle， good controllability and ease of engineering implementation. SCS-500L is a stability control device based on high speed digital signal processor（DSP） technology， having advantages of high-speed calculation， high-precision sampling， high reliability and a friendly man-machine interface and thus being capable of adapting to flexible requirements as a result of power system changes in topology and operation modes. Digital simulation results are given to validate the new method and the new device.