The 300 Mvar fast dynamic response synchronous condenser is widely used in UHVDC transmission system. In order to identify the model parameters of synchronous condenser, a step-by-step parameters identification method based on alternative iterative optimization is proposed in this paper. A six-order practical motor model considering saturation characteristics is adopted. A small disturbance test scheme including excitation step disturbance, load rejection disturbance and terminal voltage disturbance is designed. Then, based on the ADMM method and trajectory sensitivity analysis method, the steady parameters and transient and sub-transient parameters are identified iteratively using three groups of disturbance data simultaneously. The proposed parameters identification strategy is applied to the ikezhao-yinan UHVDC project successfully. The research shows that the identification result using three groups of tests is better than that using one single test, and the step-by-step identification method is better than that one-time identification of all parameters. It solves the problem that the parameters identified based on the single disturbance test cannot reproduce the other disturbance tests data, and improves the applicability and accuracy of the parameter identification results.