CNR（Carrier-to-Noise Ratio）, an important indicator to evaluate the navigation signal quality, which directly affected the global navigation satellite system receiver performances of signal acquisition, carrier tracking, and data demodulation. Based on the distortion model of ICAO 2OS(International Civil Aviation Organization 2nd Order Step), the analytical expressions between CNR estimation and the distortion parameters of ICAO 2OS threat model were induced by using both coherent early-late processing and non-coherent early-late processing. And the influence of distortion parameters on CNR estimation error was analyzed. Theoretical analysis and simulation results show that carrier-to-noise ratio only changes in the 10^-2 order of magnitude when the TMA(Threat Model A) parameter changes within the set range, and can reach about 0.1dBHz caused by the TMB(Threat Model B) parameters. When increasing the TMB oscillation frequency and decreasing the attenuation factor can increase the estimated CNR. Consequently the distortion parameters of ICAO 2OS threat model have a small influence on the CNR estimation loss. Therefore, using the output CNR of the global navigation satellite system receiver to monitor the navigation signal abnormality is not reliable.