The invention discloses a high-voltage direct-current power transmission multi-channel supplementary damping control method based on a variable structure. The high-voltage direct-current power transmission multi-channel supplementary damping control method has the characteristics that sub-synchronous and low frequency oscillation frequencies and damps, and a system reduced-order model are identified by a TLS-ESPRIT (Total Least Squares-Estimation of Signal Parameters via Rotational Invariance Techniques) algorithm with high operation efficiency and anti-interference capability; a variable structure control principle is combined and an attached controller containing a virtual state variable is designed; finally, a state observer is led in to eliminate the virtual state variable so as to realize an HVDC (High Voltage Direct Current) variable structure controller with an output feedback form; then a variable structure control theory is used for designing a multi-channel direct-current attached damping controller so as to reduce the mutual influences between oscillation modes; the sub-synchronous and low frequency oscillation can be inhibited synchronously. The method is efficient and feasible; the variable structure control theory has strong disturbance rejection on complicated changeable working conditions of an actual power grid; meanwhile, a multi-channel structure is utilized by using the controller to solve a coordinated control problem among the plurality of controllers; the invention provides a controller design method of an actual large power grid with very strong operability.