The fault current distribution of direct current (DC) transmission systems for large-scale offshore wind power presents new characteristics. To address this issue, the transient fault characteristics of the DC transmission system line in offshore wind power when a unipolar grounding fault occurs are analyzed, as well as its influence mechanism. The response process of unipolar grounding fault can be divided into three transient phases: DC-side capacitor discharge, grid-side current feeding, and voltage recovery. The mathematical expressions of fault currents in the transient phases of unipolar grounding fault in the DC transmission system in offshore wind power are derived, and the equivalent electrical model is established. According to the resistance value of the grounding resistor, the fault process is divided into over-damped and under-damped states, and the influence of the grounding resistor on the fault characteristics of the system is analyzed. Finally, a simulation model of ±10 kV DC transmission system in offshore wind power is constructed in MATLAB/SIMULINK simulation software, which verifies the correctness of the theoretical analysis of the transient characteristics of the system in the event of a unipolar grounding fault.