Abstract: The results of pipeline integrity assessment are directly affected by the reliability of quantification precision of the in-line inspection defect size, and accordingly, the determination of maintenance plan and re-inspection time will be affected.By comparing In-line inspection systems qualification(SY/T 6825-2011) with In-line inspection systems qualification(API Std 1163-2013), the definition and relationship between reliability and confidence were clarified, and the basic concept of quantitative index of in-line inspection defect size in China was revised.With reference to the relevant provisions of API Std 1163-2013, the reliability index calculation method for quantization precision of in-line inspection defect size was established based on Agresti-Coull method, and the acceptance criterion for judging the quantization precision index of inline inspection defect size provided by the in-line inspection service providers was demonstrated, providing the basis for pipeline operators to select and verify the quantification precision index of in-line inspection defect size.