White noise is one of the inherent characters of FOCT, thus the real-time sensing waveform of a small current FOCT current displays many glitches. This paper analyzes the source and the characterization method of white noise in FOCT, research in the statistical properties of white noise in FOCT while the equipment is in a zero-input current environment. Data in time-domain and frequency-domain is analyzed and compared under different primary input current. The result indicates that white noise in FOCT accords with a normal distribution with mean zero, and the noise magnitude is independent of the primary current value. On that basis, we study how the white noise affects test result of FOCT, and also provide an effective method of how to reduce the impact.