With the increase of the permeability of renewable energy, the number of power electronic devices in the power system is increasing, and the interaction between the devices will induce broadband oscillation in the frequency range from a few to thousands of Hz. There is a problem of broadband oscillation caused by direct-drive wind farms (DDWFs) connected to the grid via a voltage source converter-based high voltage direct current (VSC-HVDC). To help solve this small signal and impedance models are established based on the dynamic mathematical model of the VSC-HVDC grid-connected system with DDWFs. Then, the oscillation modes are analyzed by eigenvalue analysis and the applicability of "negative resistance" theory in the analysis of the broadband oscillation mechanism of the system is verified. Finally, the influence of parameter changes on oscillation characteristics is comprehensively analyzed using eigenvalue analysis and the impedance method, and the correctness of the above analysis is verified by PSCAD/EMTDC. The results show that parameters can be adjusted based on eigenvalue analysis and the impedance method to reduce the amplitude of broadband oscillation. This work is supported by the National Natural Science Foundation of China (No. 51507064).