Abstract: In the DC collection and transmission of offshore wind power system, the single-phase modular multilevel converter-based front-to-front (MMC-FTF) converter is the key interface device that connects the medium-voltage convergence line with the high voltage direct current (HVDC) line. However, the impedance modeling for MMC-FTF is rarely reported, and the small signal stability of HVDC systems containing MMC-FTF is still unclear. To address this problem, this paper firstly realizes the unification of the ac and DC impedance models of the single-phase and three-phase MMC under the multi-harmonic linearization method based on the frequency coupling effect and the proposed common mode and differential mode extraction matrix, and establishes the DC impedance models of MMC-FTF. Secondly, the impedance criterion is utilized to reveal the oscillation risk when the MMC-FTF is interconnected with the onshore three-phase MMC converter station. Then, the effects of different controller parameters on system stability are quantitatively evaluated based on the phase angle sensitivity index, and the corresponding parameter-tuning criteria are proposed to improve the stability of the system. Finally, the correctness of the results is verified through comprehensive simulation results and hardware-in-the-loop (HIL) experiments.