Abstract：Jack-up rigs are widely used in offshore oil and marginal oil field development. According to statistics, punch-through accidents account for more than 50% of jack-up rigs' accidents, leading often to serious damages of the jack-up rigs and casualties. Taking the Arab drill 19 jack-up and a Chinese jack-up as examples, the causes of the accidents and the seriousness of consequences are analyzed. The projected area method is used to study the possibility of punch-through for a domestic jack-up at a well site in Bohai Bay. It is found that the punch-through risk at the well site is high. The calculated penetration depth does not agree very well with the actual operational record, with a great error, which is analyzed in this paper based on new research results of punch-through. It is shown that during punch-through in a jack-up rig, a sand plug will be formed under the spudcan of the jack-up rig, which increases the ultimate bearing capacity of soil, and reduces the actual depth.