Harmonic interaction characteristics between current control loops of different units in the direct drive wind farm are studied. Based on αβ coordinate system, the Quasi-Proportional Resonance (QPR) controller is used to track the input reference current of the direct drive wind turbine. Considering the digital delay of the inverter, the capacitive current feedback active damping is used to suppress the natural resonance of the LCL filter. Combined with the wind farm topology, a multi-input and multi-output transfer function matrix model of the direct drive wind farm is established. Based on the principle of Relative Gain Array (RGA), the influence of different control parameters, equivalent grid inductance and the number of grid-connected units on the harmonic interaction between the current control loops of direct drive wind turbines in the direct drive wind farm is quantitatively analyzed. The direct drive wind farm model is respectively built on Matlab/Simulink software and StarSim+PXI hardware-in-the-loop real-time simulation platform. Simulation and experimental results verify the validity and correctness of the harmonic interaction analysis method and results. This work is supported by National Key Research and Development Program of China (No. 2016YFB0900600) and Science and Technology Project of State Grid Corporation of China (No. 52094017000W).