In directional radar the directional characteristic is affected by nonideal sum and difference performance caused by small mismatch of microwave bridge port. The microwave sum and difference network of monopulse directional radar with three channels monopulse is analyzed and the model for microwave sum and difference network is established. This model is used to simulate the impact of monopulse on the directional characteristic. The model is important not only to the directional performance analysis while microwave bridge port in siate of the small mismatch, but also to the monopulse radar simulation of anti-jamming.