The invention discloses a method and a device for determining a cyclic prefix (CP) mode and timing with orthogonal frequency division multiplexing (OFDM) symbols. The method comprises the following steps: firstly, carrying out conjugate multiplication on each segment of data in received data and data of the segment of data after delaying NFFT to obtain an r'''m sequence; secondly, utilizing a preset repetition period Nperiod as an interval to extract data from the r'''m sequence at equal intervals for accumulating, so as to obtain an r''m sequence; thirdly, dividing CP windows in the r''m sequence aiming at a normal CP mode an expansion CP mode respectively, accumulating energy in each CP window and determining an alternative initial position of the OFDM symbols by detecting the peak value of the accumulated result; and finally, determining the CP mode aiming at the peak value determined by the normal CP mode and the peak value determined by the expansion CP mode, and outputting the initial position of corresponding OFMD signals. By applying the invention, the CP mode and the initial position of the OFDM symbols can be determined, thereby helping a terminal to realize master/slavesynchronous signal detection in frequency domain.