With the large-scale application of new power electronic devices in power grid， the risk of harmonic resonance is higher and higher. However， the design of trip criterion of security and stability control （SSC） device in the past seldom considers the new harmonic condition. Combined with the actual harmonic resonance happened in CSG， this paper carries out the study of harmonic impact on SSC device. According to defects found in studies that SSC device can’t effectively filter out specific frequency of high order harmonic， sampling algorithm of the device is analyzed theoretically， and harmonic impact of no-fault line trip criterion is also analyzed under existing conditions. Combined with the actual SSC devices in CSG， a temporary measure to deal with the risk of refusing action of SSC device is put forward. Also an ultimate optimization method of SSC software and hardware is proposed and tested on the SSC test platform. Test results verify the effectiveness of the proposed method， and meanwhile the method has important significance to the design and improvement of other SSC devices.