The stability of a wind turbine grid connected inverter and power transmission system is an important component of the stable operation of a large-scale offshore wind power generation system. A method of optimization design of controller parameters for offshore wind power system considering small signal stability is proposed. First, based on the harmonic linearization principle, a sequence impedance model of a direct-drive wind turbine (PMSG) grid-connected inverter and a high voltage direct current (HVDC) transmission system with diode rectifier (DR) is derived. Then, the characteristics of a DR-HVDC interconnected system are analyzed and the applicability of a stability criterion is discussed. The control bandwidth and damping ratio range of the grid-connected inverter control system are determined from the perspective of the controller. From the system point of view, we consider the effect of the right half plane zero-pole and control parameters on the impedance ratio of the interconnected system above. Finally, the parameter optimization design process of the control system for the grid-connected inverter of the PMSG is given. Based on Matlab/Simulink, the time domain simulation model of offshore wind power through the DR-HVDC DC transmission system is established. The simulation results verify the correctness of the theoretical analysis and the effectiveness of the proposed method. This work is supported by the National Key Research and Development Program of China (No. 2021YFB2400900).