This paper describes a novel ultra wideband microstrip directional coupler, which adopts a broadside coupling theory. Compared with the narrow coupling structure, broadside coupling structure can increase coupling area and realize 3 dB tight coupling in a wide frequency range. In order to further expand the operating bandwidth of the coupler, this paper uses tapered coupling structure instead of the traditional rectangular couping structure. The HFSS simulation results show that the insertion loss of the directional coupler is less than 0.9 dB, the return loss at the input port is more than 21 dB and the isolation at port is more than 20 dB across 2 GHz to 6 GHz.