The wellbore of an offshore exploratory well above the mudline can be considered as a long slender member bar. High casing running may lead to wellhead damage and instability. It is crucial to calculate the casing pipe’s running velocity. Based on the wellbore configuration, this paper conducted mechanical analysis of the wellhead and wellbore string. According to compressed bar stability theory, wellhead and wellbore bearing capacity calculation models were established. Based on the energy conservation principle, impulsive load calculation models were set up. Setting conductor-bearing stability, wellhead strength, wellbore instability and weak formation’s fracture pressure as constraint conditions, the maximum casing pipe running velocity models were derived. Taking a Bohai Basin exploration well as a computational example, this paper analyzed the influence of casing running velocity on wellhead loading and wellbore stability. After being applied in exploration wells, this theoretical model was proved to be significant for offshore drilling design and site operation.