Abstract: With the efficient development of offshore oil and gas, the extended well testing has become an important means to obtain production capacity parameters and evaluate oil and gas reservoirs accurately. The extended well testing has long operation cycle and it may undergo the harsh sea conditions, so the stability of riser is a key factor to ensure the operation safety. To study the stability of riser under the harsh sea conditions, we analyzed the stress characteristics of the riser in the process of the extended well testing with the casing program characteristics of the extended well testing well as the beginning point. Then, the mechanical model of the riser was established. It is revealed that the maximum equivalent stress under the extreme load occurs near the water surface, where the riser is the dangerous point of the extended well testing string. Finally, based on the sea conditions in the East China Sea and the geological conditions of the well, the geometric parameters of the riser in the extended well testing well were designed and the mechanical model for the riser in the process of extended well testing was established by using the finite element simulation software ANSYS. In addition, the mechanical characteristics of the riser were calculated by applying the environmental loads and constraints to the model, and then compared with the on-site measurement to verify the reliability of the new model.