Narrowband internet of things (NB-IoT) is a new cellular technology introduced by the 3rd generation partnership project (3GPP) for the purpose of massive connections. NB-IoT devices are expected to have low cost and low power. This paper proposes a new low-complexity algorithm for cell identity detection based on the property of the synchronization sequences. A two-stage grouping algorithm is presented to divide all the synchronization sequences into groups by utilizing the property of Zadoff-Chu sequences. Therefore, the number of operations are reduced and hence the computational complexity is decreased. The simulation results show that the proposed grouping method can achieve 70 percent reduction with slight performance loss. Index Terms - NB-IoT, cell ID detection, synchronization, Zadoff-Chu sequence.