Abstract: A helium cryogenic system is constructed in Southern Advanced Photon Source test platform, to provide 100 W@2 K vertical and horizontal test modes for the superconducting cavities. The decompression cooling pumps are adopted to achieve 2 K environment by vacuum equipment, which are consist of three mechanical pumps and Roots pumps. To achieve the requirements of 2 K@31.3 mbar±0.1 mbar and retain stable liquid level, a set of control logic is designed for decompression cooling process. Through the cooperation including the controllable opening of the vacuum unit entrance butterfly valve, variable frequency control for the Roots pumps, and the opening of the bypass valve, the conversion process from 4 K to 2 K can be achieved within 2 hours in multiple rounds of test, with the application of this logic, while the waiting time during test can be greatly reduced, and the pressure fluctuation can be controlled within 31.3 mbar±0.1 mbar. The continuous and stable test environment for the superconducting cavity testing is provided, and the stable operation duration is up to 720 hours in one test round.