To recognize modulation in MIMO signals, a modulation classification method based on blind source separation is proposed. Singular value decomposition of the received signal’s correlation matrix is used to estimate the number of transmitting antennas and whiten the received signal. The source signal is recovered with joint approximate diagonalisation of eigen-matrices (JADE). Based on analyses of four new spectral features and the fourth-order cumulant, a modulation recognition algorithm is proposed for six kinds of MIMO signals. The method does not need to estimate the carrier frequency and the symbol rate, thus is highly practical. Simulation results show that the proposed method has good performance.