Abstract: Hard spots in pipelines refer to areas where the localized hardness exceeds a specified standard. Under stress, these spots are prone to fatigue cracking, sulfide stress corrosion cracking (SSCC), and stress-oriented hydrogen-induced cracking (SOHIC), posing serious threats to the safe operation of the oil and gas pipelines. This study focuses on establishing an eddy current-based detection method to identify hard spots accurately. This method is based on an eddy current detection model and an analysis of the factors influencing eddy current impedance signals, such as material conductivity and magnetic permeability. Additionally, the study examined how defects of varying sizes and different stress levels affect the detection results. The relationship between material stress and relative magnetic permeability was also explored. Hard spots in pipelines were simulated using Q235 steel plates subjected to quenching, and a series of experiments were conducted to verify the effectiveness of the proposed detection method. The experimental results indicate the following: ① The presence of hard spots leads to a decrease in eddy current impedance, with the impedance value at the hard spot locations being significantly lower compared to other areas on the steel plates. ② Eddy current detection results show only slight changes when using steel plates with small-sized defects, indicating that these defects have minimal impact on hard spot detection. ③ The eddy current impedance value was significantly reduced when using steel plates with large-sized defects. ④ Hard spots and defects can be distinguished through a comparative analysis of signals generated at different detection frequencies. ⑤ The impedance at the hard spot locations on the steel plates exhibits an approximately linear relationship with stress, specifically increasing with elevated stress levels. In conclusion, the utilization of the eddy current detection technique enables the detection of hard spots in pipelines and the monitoring of stress at these locations. The findings of this study provide a theoretical basis for subsequent experiments.