Abstract: Based on the concept of optimum detection theory and optimum spatial filtering theory, the expression of OSF (optimum spatial filter) in frequency domain is derived. The realization of OSF in the case of real target radiated noise in the background of Gaussian white noise for equal spaced line array is illustrated. The beam patterns of CBF (conventional beamforming) system and OSF system are compared. The analysis of at sea experiment data shows that the OSF described in this paper can effectively suppress multiple interferences, which come from in the neighbourhood of target direction and extract the weak signal. This kind of OSF can be easily implemented in digital sonar system.