Waveguide invariant is widely used in underwater target passive ranging. In shallow water with uniform sound speed profile, the value of waveguide invariant is approximately a constant,while in shallow water with thermocline, it varies in a wide range.The waveguide invariant distributions and striations in these two conditions are analyzed respectively. On the basis of wavenumber difference between reflected modes and refracted modes, a wavenumber-frequency domain filtering technique is proposed to separate the two groups of modes. The required relationship between array element space, total array length and target azimuth for effective application is discussed. Finally, the simulation results indicate that in shallow water with a thermocline, refracted modes can be effectively filtered out using the wavenumber-frequency domain filtering technique and the target’s range is estimated accurately. Waveguide invariant is widely used in underwater target passive ranging. In shallow water with uniform sound speed profile, the value of waveguide invariant is approximately a constant, while in shallow water with thermocline, it varies in a wide range. Waveguide invariant distributions and striations On the basis of wavenumber difference between reflected modes and refracted modes, a wavenumber-frequency domain filtering technique is proposed to separate the two groups of modes. The required relationship between array element space, total array length and Finally, the simulation results indicate that in shallow water with a thermocline, refracted modes can be effectively filtered out using the wavenumber-frequency domain filtering technique and the target’s range is estimated accurately.