Abstract: Stable operation of the electrolytic oxygen system is key to stable continuous oxygen supply inside human spacecraft， and the critical operation condition of the electrolytic system is the basis of safe and stable operation. The critical water flow rate and the critical operating current for the operation of a single cell of an electrolytic oxygen generating core and an electrolytic stack under microgravity conditions are investigated， and a water supply optimization scheme is proposed by improving the local structure of the electrolytic stack. Numerical simulations are carried out using fluid dynamics simulation software， and a gas-liquid mixed two-phase flow model is used to describe the flow process inside the electrolytic core. For both the electrolytic cell unit and the electrolytic stack， there is a limit water flow rate when the operating current is a constant. When the water flow rate is smaller than this critical value， a local water shortage phenomenon occurs inside the core. For a specific water flow rate， there is a critical operating current， and when the current is greater than the critical value， there is a local water shortage inside the core. The uniformity of the flow distribution of the multilayer core can be significantly improved by increasing the diameter of the water supply pipe. With the increase of the diameter， the ratio of the minimum water mass flux to the average mass flux increases linearly and the ratio of the maximum flow rate to the average flow rate decreases linearly. Typically， when the pipe diameter increases by 75%， the ratio of the minimum flow rate to the average flow rate increases by about 39% and the ratio of the maximum flow rate to the average flow rate decreases by about 30%. The uniformity of water distribution is better when the inlet and outlet are on the same side of the electrolysis stack， and the ratio of the maximum to the average water flow can be reduced by more than 30% with the same water flow rate. The research work can provide parameter suggestions for the safe operation of electrolysis tanks for human spacecraft and a basis for the optimal design of the system.

Key words: water electrolysis, critical water flow rate, critical working current, microgravity, two-phase flow