Abstract: It is important to control the space station microbial populations in the Water Processor Assembly for astronauts' drinking water and living water. Excessive microbial quantity will affect the astronaut health and some sulfate-reducing bacteria attached on the pipeline equipment and the chemical reaction of H₂S gas will corrode the equipment. The control of the water circulation treatment system of microorganisms involves three aspects:1) the ground control; 2)the use of special materials in orbit; 3) the sterilization. In 2003, the NASA issues a series of requirements about the microbials in the ISS. This paper puts forward two microbial detection methods: the ATP bioluminescence and the method of measuring the iodine number and the silver ions. By comparing the advantages and disadvantages of the two methods and considering many respective aspects, it is shown that the monitoring the amount of iodine and silver ions is an efficient way to control the amount of microorganisms.