Abstract It is generally acceptable to use soft string model for simple wells and stiff string for high dogleg wells. But these models fail to reveal the complex mechanical behaviors of tubular string in tripping in, tripping out, and sliding drilling operations with time as some segments of tubular string may move upward and downward alternatively. As a result, the model under predicts or over predicts the behaviors of tubular string, and requires multiple calibrations of friction factor. To avoid this, a new model has been developed to estimate dynamic torque and drag. In the new model, the effects of tubular vibration, drill fluid, viscous force, and velocity-dependent friction force are all considered, which overcomes the shortcomings of all previous models. The new model can be used for calculating dynamic mechanical behaviors of tubular string under tripping in, tripping out, and sliding drilling operations. The results provide new insights that have not been previously presented and will provide valuable insights into well planning and designing.