Effective C/N₀(carrier-to-noise ratio) is an important index of GNSS (global navigation satellite system) receiver data demodulation and tracking performance. During past years, analysis of interferences effects on effective C/N₀ have been put forward. However, effective C/N₀ limits analysis hasn’t been presented. Thus, effective C/N₀ limits were analyzed. The theory model of effective C/N₀ and BER (bit error ratio) were derived in the presence of CWI(continuous wave interference), and results show that effective C/N₀ fails to evaluate data demodulation performance under specific condition. Then, the mechanism of effective C//N₀ malfunction was elaborated and the application condition of effective C//N₀ was presented. Finally, theory analysis is verified by simulations.