A new optimal engine crankshaft torsional vibration dampers （TVD） design concept and method are proposed to reduce the torsional resonant responses of the crankshaft. The proposed method derives from the modal inertia method is used to simplify the multi-degree of freedom system of a crankshaft to a single degree of freedom system，then the traditional Den Hartog's optimal tuning theory is extended to the off-tuning theory，and the negative and positive off-tuning concepts are proposed. Furthermore，in view of the occurring of torsional resonances are produced by multiple excitation frequencies of a crankshaft，an optimization model is developed to maximize the linearity of the torsional vibration response of the engine crankshaft based on the proposed off-tuning theory. In this model， the adjustment factor of the natural frequency of the TVD is chosen as the optimization design parameter. The TVD optimized by the method is shown to have a much better Overall result and a best matching between TVD and crankshaft. Through testing，the torsional vibration responses of Hub and accelerating noises of engine are proven to reduce after optimization， and the noise，vibration and harshness （NVH） performance of the engine is improved.