For the detection of unresolved targets on the planar array radar, a rectangle array was taken as example to analyze the statistical distribution of imaginary of complex monopulse ratios in azimuth and elevation firstly. The detection model was given for planar array and an ellipse decision domain for the imaginary part of the monopulse ratios was set according to the constant false alarm theory. Besides, the real part of the monopulse ratios was used to assist the decision. By combining the two decisions, the detection algorithm for unresolved targets based on complex monopulse ratio was proposed. Simulation results based on the rectangle array for two targets demonstrate that the proposed algorithm performs well in the conditions of high signal to noise ratio, wide angle separate and suitable complex amplitude ratio.