Abstract: Aiming at the influence characteristics of underwater channel multipath transmission, an image transmission method based on improved frequency shift keying (FSK) technology is proposed to effectively solve the problem of tailing overlap caused by pulse switching of the carrier data in underwater transmission. The FSK data frame structure with carrier interval protection is applied to adapt to the signal transmission environment in the underwater. The monochrome image data are encoded into frames, processed with direct modulation, and then transmitted through the underwater acoustic transducer, which effectively improves the system's anti-interference and data transmission capability. The tests show that, when the transmission rate and the carrier duration are determined, i.e., the transmission rate is R_b and the duration is τ, the corresponding protection interval between adjacent carriers should be not less than 2τ, and the system can realize stable data transmission. In addition, the restored image has a similarity of 98.3% compared with the gray histogram simulation, and this indicates that the system can basically realize zero bit-error-rate reduction.