Abstract: In high-energy laser systems, the performance parameters of large-aperture sampling optics determine the accuracy of beam testing and evaluation, as well as the precision of overall system performance control. This paper focuses on the performance testing requirements of sampling optics with high-reflectivity (HR) on the front surface and anti-reflectivity (AR) on the back surface. Utilizing the cavity-ring-down (CRD)-based reflectivity uniformity testing of large-aperture sampling optics, the reflectivity distribution, optical loss, and high-resolution scanning imaging of defects of sampling optics are obtained by scanning measuring the incident light from both the reflective film surface and the anti-reflective film surface, respectively. Furthermore, by comparing and analyzing the defect distribution maps, the classification of defects in the reflective film, transmissive film, and substrate of the sampling optics is achieved. Finally, by establishing a dual channel CRD system, the residual reflectance distribution of anti-reflective film and the types of defects in the transmissive film were obtained. The testing and analysis method proposed in this paper provides a systematic and comprehensive characterization tool for the performance evaluation and defect analysis of sampling optics.