Abstract: For the eigenstructure-based beam-forming method,the source number estimation is crucial to divide the signal subspace and the noise sub-space. The normal method is effect at about 0dB,which is not high enough for the practical applications. Through analyzing the space power spectrum and the correlative function of a line array,the concept of beam eigenvalue,which is decided by the line array shape,is proposed. The method based on the beam eigenvalue is designed to estimate the source number inside a beam. The simulation experiment shows that this method can differentiate the single and double targets at the lower signal to noise rate of -10dB.