This paper presents a miniaturized L-band microstrip line directional coupler loaded with double branches. Power cancellation technology is used to increase the isolation of the directional coupler, and two loading branches with different lengths are used to reflect signals at two similar frequency points respectively, so as to realize the bandwidth expansion of the coupler; at the same time, the slow wave characteristics of the defective ground structure are used to reduce the physical size of the directional coupler, and the notch generated by the defective ground structure is used to combine the two notch waves generated by the reflection of the two branches to further improve the bandwidth and isolation of the directional coupler. The coupler works in the L-band, the coupling is about 10 dB, the isolation in the entire L-band is better than -20 dB, and the maximum is about -52.81 dB, and the fractional bandwidth is 60%. Finally, the designed directional coupler is processed and tested, and the test and simulation results are in good agreement, which proves the feasibility of the circuit.