Source depth estimation in the direct arrival zone of deep water can be achieved by matching the multipath arrival angles, arrival time delays, or interference cycles in the frequency domain. A vertical array with a high spatial spectrum resolution or a source with a sufficient bandwidth is generally required for these methods. This paper proposes a depth estimation method for line spectrum sources, though matching the measured interference structure in the arrival angle domain obtained from the beamforming of a vertical line array, with the modeled interference structure under different assumed source depths. The effectiveness of this method is verified by a towed source data collected by a bottom-moored vertical linear array in the South China Sea.