Accurately identifying karst channels between injection and production wells is a key issue to the treatment of water channeling in fracture-cavity reservoirs，expansion of waterflood swept volume，and further achievement in balanced displacement of such strongly heterogeneous reservoirs. Taking karst fracture-cavity reservoirs in the palaeo-subterranean river as the research object，we propose a method for identifying karst channels in such reservoirs. Through comparison between the results obtained respectively from the time domain simulator and the frequency domain simulator，the accuracy and high efficiency of the latter are verified，and the formation characteristics reflected by the parameter changes in karst channels are obtained. Research shows that harmonic pressure response analysis can identify karst channels with resistance to interference，low cost，and high accuracy. Pressure is transmitted in fracture-cavity reservoirs mainly via karst channels. For head responses at different positions，the amplitude drops and the phase shifts to different degrees. Through comprehensive analysis of amplitude reduction and phase shift of the head response at each position to the harmonic signal，we can identify the area where the test point is located. The results show that for the response with an amplitude of almost 0，the test point is located in matrices；for the amplitude response with slight phase shift，the test point is located in karst channels；for the amplitude response with pronounced phase shift，the test point is located in the matrices close to karst channels.