The statistical property of monopulse ratio was derived under the condition of single target and multiple unresolved targets. According to the inherent difference, the multiplicity detection algorithm was proposed using complex monopulse ratio. Taking two targets as an example, the detection performance was presented by Mont Carlo simulations, with respect to the SNR (signal to noise ratio), the angle interval and the complex amplitude ratio. Some meaningful conclusions are inferred qualitatively. When one target predominates, the detection performance is low. As for two comparative targets, the detection performance is improved with the increase of the phase difference. This algorithm can eliminate the outliers of estimated angle effectively and can lay the foundation for the resolution and measurement process of multiple targets.