To realize the on-line measurement of power, frequency and phase of modular relativistic triaxial klystron amplifier, a compact directional coupler with high directivity and bandwidth is simulated and experimentally studied. Based on the theoretical analysis of the pinhole coupling theory and the phase superposition principle, a dual-hole compact directional coupler is designed. On this basis, the main and auxiliary waveguides are connected orthogonally, and the coupling holes are distributed along the axial and angular directions, which further shortens the length of the coupler. The parameters of the coupler are optimized by electromagnetic simulation. The simulation results show that when the center frequency is 10 GHz, the coupling degree of the ordinary dual-hole directional coupler to the TM₀₁ mode is -60.68 dB, and the directivity is greater than 20 dB in the bandwidth of 250 MHz, and the coupling area length is 3.49 cm. The coupling degree of the improved directional coupler to the TM₀₁ mode is -58.1 dB, and the directivity is greater than 20 dB in the bandwidth of 300 MHz. At this time, the length of the coupling zone is only 1.8 cm (about 0.6λ）. The cold cavity experimental results of the coupler are in good agreement with the simulation results.