The degenerate unmixing estimation technique （DUET） algorithm is a typical underdetermined blind source separation algorithm. However， as a binary time-frequency mask-based method， DUET erroneously results in some interference signals retention. This paper proposes an improved DUET algorithm based on two-step single-source point screening. The cosine angle algorithm is used for the first-step single-source point screening， and then a similarity calculation method is employed for the second-step single-source point screening. After obtaining more accurate target and interference signals through two-step single-source point screening， the filter designed to cancel the interference signals replaces the binary time-frequency mask of DUET， achieving interference signal suppression and target signal extraction. Simulation results show that the proposed method has good performance in both positive definite and underdetermined blind source separation.