The invention discloses a synchronous detection device applied to a narrowband wireless communication system terminal. The synchronous detection device comprises a downsampling module, a sliding autocorrelation module and a differential cross-correlation module. The downsampling module downsamples a received 1.92 MHz time domain signal into a 240 kHz signal according to the OFDM symbol duration and a fixed downsampling pattern. A sliding autocorrelation module moves sampling point data in a 10ms wireless frame processing window bit by bit to serve as an initial position t, takes the duration of 11 OFDM symbols as a sliding window, and performs correlation accumulation on 11 groups of sampling points in the sliding window at intervals of two groups. A differential cross-correlation module differentiates the obtained preliminary synchronization sequence into two approximately equal parts, and performs cross-correlation accumulation with a locally generated main synchronization sequence.The synchronization detection device can still better detect the main synchronization signal under the application scenarios of low signal-to-noise ratio and large frequency offset of a narrowband wireless communication system, and is lower in implementation complexity and low in cost.