In order to solve the problem of MIMO radar underdetermined blind separation under the condition of overlapping in time domain, frequency domain and time-frequency domain, a blind separation algorithm based on sparse smooth reconstruction and ridge estimation is proposed. Firstly, underdetermined blind signal separation is simplified to signal reconstruction with known mixed matrix. Secondly, using the time-frequency sparsity of MIMO radar signal, a time-frequency smooth sparse reconstruction algorithm is proposed, and the time-frequency information is modified by the ridge estimation method. Finally, the time-frequency source signal is recovered by inverse transformation. The algorithm can reconstruct the coding information of source signal and radar signal at the same time, which provides a new way to solve the problem of radar signal separation in complex electromagnetic environment.