The invention discloses a method and system for extracting a corona current signal of an ultra-high voltage direct current transmission line. The method includes the following steps that original dataof corona current at multiple voltage levels truncated according to a given length is obtained, and a data matrix is generated; according to the data matrix of the original data of corona current truncated at 0 kV, based on the K-singular value decomposition (K-SVD) and the non-negative OMP algorithm, a dictionary set corresponding to the original data of corona current truncated at 0 kV is established; the non-negative OMP algorithm is adopted, and according to the non-negative sparse expression coefficient on the dictionary set, an updated data matrix of the original data of corona currentat multiple voltage levels is built; according to the updated data matrix, based on the K-SVD and the non-negative OMP algorithm, a total dictionary set corresponding to the original data of corona current at multiple voltage levels is obtained. For the total dictionary set, decomposition and reconstruction of the original data of corona current are performed through the non-negative OMP algorithm, background noise is removed and the corona current signal is obtained.