The invention discloses a method and a device for coarse synchronizing sub-frame, comprising the following steps: conducting hard decision on the sampled signal by terminal sampling digital base band signals, thus obtaining the sign bits of the sampling signal; intercepting the virtual sub-frame in the sign bit of the sampled signal by the terminal; carrying out the differential operation on the adjacent sample points in each intercepted virtual sub-frame, thus obtaining the differential hard decision virtual sub-frame data; carrying out accumulation on a plurality of differential hard decision virtual sub-frames and removing the sign bit at each sample point of the accumulated result by the terminal; and searching the position with downlink synchronous code characteristic in the accumulated result by terminal. In the invention, based the design of hard decision data accumulation, AGC (Automatic Gain Control) attempt is avoided; the differential operation is carried out so as to eliminate the influence of phase and channel, thus eliminating the dependency of the coarse synchronization of district searching on a plurality of factors such as radio frequency devices, ADC (Analog to Digital Converter) bit-width and dynamic scope of downlink signal and the like, thereby obviously improving the stability during the coarse synchronization process of district searching.