Low-temperature refrigerating cascade systems are widely used in preservation of biological issues, cooling detectors and providing circumstances for material and component testing. Contrast to some traditional cascade cycles, this paper presents an auto-refrigerating cascade cycle that employs a rectifying column. The rectifying column is used to separate high boiling component and low boiling component. A thermodynamic analysis of the cycle is conducted with R134a/R23 as working fluid. Energy balance equations are established for each component of the system. Some important factors, including composition of the mixture, suction temperature of the compressor and operation pressure of the rectifying column, are analyzed of their impacts on compression ratio and coefficient of performance. The generalized Patel-Teja equation is used to calculate VLE and related thermodynamic properties.