Microstructure and low-frequency noise test were measured for N80 steel casing drilling, through SEM analyzed their morphology. Results showed that: (1) power spectral density of 1/f noise increases two orders of magnitude after fatigue crack growth. (2) 1/f noise parameters of γ and B are significantly increased, indicating that the process of fatigue produced more cracks, defects, and combination centers, which were proved by microstructure morphology. From the mechanism of fatigue crack growth of N80, defects and cracks resulting from fatigue are the numbers of kind of fluctuations. In essence, low-frequency noise is a type of fluctuations, which can serve as a viable tool to study the defects and the characterization of defects.