Abstract: Based on the vibration measurement of some piled offshore platforms in the Bohai Bay, an approximate method for calculating the fundamental frequency of platform is suggested. The horizontal stiffness of platform is composed of stiffness of jacket in series with stiffness of piles. The stiffness of piles is computed from the theory of beams on elastic foundation in which the base reaction coefficient increases linearly with the depth, and the ratio constant is denoted by "m". The stiffness along the height of jacket is very different. The stiffness of tidal section is the smallest, so that it affects extremely the stiffness of jacket. For calculating stiffness of this section, an elastic hinge model is presented. The comparison between the result computed by approximate method for a four-leg platform with that computed by electric exact method shows that the discrepancy is less than 5%. The computed results of some platforms show that the stiftness of piles affects significantly the freqgency. If "m" takes the value of 200T/m⁴ or 300T/m⁴, the computed frequencies agree well with the measured values. Therefore, authors recommend that in the preliminary design, "m" may take the value of 200t/m⁴. The fundamental frequencies of platform in shallow water range from 1 Hertz to 3 Hertz, which coincide with forced frequencies excited by ice, mud pump and earthquake. The forced vibration of platform should be further investigated.