The dynamic characteristic of gas-liquid two phase flow in the recessed region of a gas-liquid coaxial injector was investigated with the frequency method.Considering the liquid-phase as a lumped-element,the dynamic response of the pressure drop fluctuation in the recessed region to the liquid flow fluctuation was analyzed with the homogeneous flow model.The dynamic characteristics of the injector models were calculated.The calculation results show that,the amplitude of pressure drop fluctuation and the phase-delay between the pressure drop fluctuation and liquid flow fluctuation will increase with the rising fluctuation frequency.A smaller length to diameter ratio recessed region and liquid injector mass flow,or a higher ambient pressure and larger gas to liquid ratio can depress the pressure drop fluctuation in the recessed region.