The invention relates to an on-line identification method for dynamic torsional stiffness of an elastic coupling unit. The method comprises: a torsion vibration signal of a shafting of a diesel engine is collected and an inherent frequency experiment value of torsion vibration of the shafting is obtained; on the basis of a model, calculation is carried out to obtain an inherent frequency calculation value of the torsion vibration of the shafting; a relative error between the inherent frequency calculation value of the torsion vibration of the shafting and the inherent frequency experiment value and a 2- norm of the relative error are calculated and a torsional stiffness correction function of an elastic coupling unit is constructed; a 2- norm of a relative error in a current iterative loop step and a 2- norm of a relative error in a previous iterative loop step are compared and the 2- norm of the relative error in the current iterative loop step is compared with a preset error limit, torsional stiffness correction of the elastic coupling unit is carried out, and a dynamic torsional stiffness value of the elastic coupling unit is outputted to realize on-line identification of the dynamic torsional stiffness value of the elastic coupling unit. Therefore, on-line identification can be carried out when the elastic coupling unit is not dismounted; the method can be carried out rapidly and precisely; the operation becomes simple and conveniently; the method is easy to implement; and no special experiment test stand is needed.