Abstract: The device for helium leak detection of fuel rods is one of the core components in the production process of fuel rods, which is crucial for the production quality of fuel rods and the stable operation of nuclear power plants. The current leak detection device utilizes a horizontal testing method, where fuel rods are batched for testing, and the efficiency of leak detection needs to be improved. This paper introduces the development of a multi-stage differential vacuum system for online pass-through leak detection of fuel rods. Based on the system’s characteristics, the design concept and process of the system are proposed. The method involves iterative calculations for extreme differential ratio, pumping speed, and steady-state pressure calculation. The parameters of the system are then analyzed. The research shows that the development of the multi-stage differential vacuum system is feasible. The pressure of the leak detection chamber can be maintained at a low level, the response speed of leak detection is fast, the leak detection efficiency is superior to existing equipment, and the system redundancy is high, leaving greater room for cost reduction. This system can achieve helium mass spectrometry leak detection for individual fuel rods, providing a foundation for quality control and traceability of fuel rods.