The linear zone of the S-curve, which is formed with the digital beamforming (DBF) method, is extremely narrow for monopulse angle measurements with distributed Multiple-Input and Multiple-Output (MIMO) radar. Based on the transmitting-receiving beam pattern without gratelobe, the DBF method is proposed to form the ΣΔ-beam by using the absolute value of the echo signal. The ideal null depth can be achieved and the linear zone of the S-curve is expanded because the main lobe of the Δ-beam is enveloped by the Σ-beam. The monopulse angle for a target with high angle velocity was estimated. When the Signal-to-Noise Ratio (SNR) was higher than −15 dB , the accuracy of the proposed method was better than that of conventional methods. When the SNR was equal to −15 dB , the accuracy of the proposed method was similar to conventional methods that use coherent integration before taking the absolute value of the echo signal.