Abstract: Based on 1642 records of 31 earthquakes in China from 2003 to 2015, the timeliness and reliability of OBE alarm strategies used in 2^nd and 3^rd GNP were compared and analyzed. The results showed that, for the same ground motion, when OBE was beyond limit for both 2^nd and 3^rdGNP, the OBE alarm of 3^rd GNP were later than that of 2^nd GNP, with an average delay of 20 seconds. Peak ground acceleration could be as high as 0.7 g before 3^rd GNP alarmed. OBE over-limit determination performed using data of 108 stations from the region of Ⅵ intensity of 4 destructive earthquakes showed that the proportion of stations exceeded limit among 3^rd GNP, 2^ndGNP（CGN）, and 2^nd GNP（CNNC） were 41%, 63% and 76%. It indicated that due to the aim of lowering the rate of unnecessary shutdown caused by near-field earthquakes with small magnitude and large amplitude pulses and earthquakes with small amplitude and long duration ground motions, two-parameter decision method was applied in threshold selection, thus the timeliness and reliability of 3^rd GNP was reduced, which is insecure for important nuclear facilities.