The 300MVar fast dynamic response synchronous condenser is widely used in UHVDC transmission systerm. In order to obtain synchronous condenser parameters, this paper proposes a step-by-step parameters identification method based on alternative iterative optimization. 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/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 identify all parameters at one time. It solves the problem that the parameters identified based on the single disturbance test can not reproduce the other disturbance tests data, and improves the applicability and accuracy of the parameter identification results.