With the rapid development of offshore wind power, flexible DC transmission technology has gradually become the preferred solution for offshore wind power systems. However, their safe operation still faces challenges. Real-time simulation technology has attracted more and more attention. It can simulate various complex operating conditions in a low cost and fast way. The DC/DC collection system is the key device in offshore wind farms, and its complicated cascade structures, the large number of components and the higher switching frequencies bring great challenges to the real-time simulation. In this paper, a real-time simulation method for a DC/DC collection system in an offshore wind farm is proposed, one which realizes the small-step real-time simulation of large-scale DC/DC collection systems in two steps: parallel design of the simulation algorithm and equivalent modeling of the system. First, the EMTP algorithm is redesigned as a parallel simulation algorithm for real-time simulation. The equivalent model of the submodule of DC/DC collection system is built based on a parallel real-time simulation algorithm. Second, the multi-port equivalent model of the DC/DC collection system is built based on the series coupling relationship of the submodule. Finally, the real-time simulation of the DC/DC collection system in various operating conditions is carried out on the CPU-FPGA heterogeneous real-time simulation platform. The results are compared with offline simulation in PSCAD/EMTDC to verify the accuracy and efficiency of the proposed real-time simulation method.