Suction caissons have been widely used in offshore structures. Method of installation analysis is very important research subject. This paper provides the characteristic of the seepage field of the foundation during suction installation

and gives relationship between the ratio of the suction descending on the inside wall to the overall suction versus penetration depth-diameter ratio (?1).The pump effect on ?1 is also analyzed and evaluated. A simple and convenient calculating method of both suction caisson penetration resistance and ultimate suction in sand ground is proposed. An equation of ultimate penetration depth-diameter ratio (UPDR) calculating of suction caisson in sand is proposed. The UPDR in sand with density from loose to extremely dense is analyzed; and it is about 2.1 to 2.5 while the effort to UPDR of the deadweight of the structure and the pumping effort to UPDR are not included in it.