Present research on strong motion records show that the characteristics of offshore ground motions are obviously different from those of onshore ground motions, but it is difficult to further determine the influencing factors of offshore ground motions. Based on previous research, the differences of ground motion characteristics between adjacent offshore stations during the same earthquake were compared and the reasons were analyzed. The strong motion records from six offshore stations and adjacent onshore stations with different site conditions were selected in the K-NET of Japan. The V/H PGA ratio, amplification factor spectrum, and V/H spectral ratio of strong motion records from different stations were analyzed, then the ground motion characteristics from six offshore stations and the difference of ground motion characteristics between offshore and onshore stations with different site conditions were studied. The results showed that:(1) there are obvious differences in offshore and onshore vertical ground motions;(2) the ground motion characteristics of different offshore stations, which are greatly influenced by site factors, also have great difference and obvious regularity;(3) the characteristic period of offshore horizontal seismic response spectrum is large, and the spectral curve is between those of medium hard-soil and soft-soil sites.