To improve the harmonic suppression ability of series-connected diode multi-pulse rectifiers in an offshore wind power grid-connected HVDC system, a series-connected 36-pulse rectifier based on a dual passive injection circuit at the DC side is proposed. The rectifier adopts two auxiliary passive injection circuits, which generate voltage injection harmonics after current modulation, and which can double the AC input voltage waveform from 12-step to 36-step. The principle and characteristics of dual passive injection circuits are analyzed in this paper. The input voltage expression at the AC side of the rectifier is derived, and the turn ratio parameters of the injection transformers are designed to minimize the voltage harmonic distortion rate. Finally, the fault-tolerant capability of auxiliary passive circuits with diode open circuit faults is discussed. Theoretical analysis and simulation results show that the proposed rectifier has the advantages of strong harmonic suppression, simple structure, high reliability, and strong robustness, and is more suitable for high-voltage and high-power application. This work is supported by the National Natural Science Foundation of China (No. 51867012 and No. 52067013).