The tubular mechanical behavior is very complicated during the deepwater drilling operation without riser, which is one of the key technological problems in the deepwater drilling engineering. Therefore, it is important to study the tubular mechanical behavior under loading conditions in the drilling operation, the deepwater environment and the vessel movements for safety of the deepwater drilling operation without riser. The transverse and longitudinal bending deformations, the longitudinal vibration of the drilling tubular string and the landing string design method for the deepwater drilling operation without riser are discussed in this paper. The variations of the tubular string deformation and the longitudinal vibration are analyzed by considering the relevant influencing factors. Based on the above analysis, the complicated engineering problems related to the tubular mechanics in the deepwater drilling operation without riser are simplified to be considered in the actual design and control. The results show that it is dangerous for the tubular string in the deepwater drilling operation without riser to work under the conditions of high or low axial tension force, deep water, severe vessel offset or heave and high speed current. The operation modes and the environment loads should be taken into account for the tubular string design and the strength check in the deepwater drilling operation without riser.