When drilling into a stratum that is prone to collapse in shallow sea coring drilling， it is necessary to install casing to seal the stratum to ensure mud circulation and to continue drilling at smaller diameter. The conventional casing installation method is used to install casing from the drilling ship with both the drill floor platform deck and the casing platform deck； while for the drilling ships without the casing platform deck， it needs to be temporarily installed if casing must be installed. But installation of the casing platform is limited by the volume and mass of the drill rig and drill string. In order to solve the problem of installing casing in offshore drilling and coring with drilling ships without casing platform decks， a casing running process for installing multi-tier casing in offshore drilling is designed， where telescopic casing and related equipment is used to establish a mud return channel from the subsea wellhead to the small drilling ship to isolate unstable formations， and realize mud recycling and drilling heave compensation. In the drilling process， the telescopic casing can also provide guiding and positioning to prevent the drilling tool from influence of changes in tidal current flow speeds， saving time and cost for shallow sea drilling.