With the product water from a one-stage reverse osmosis (RO) process as the feed water, ultrapure water has been prepared by a self-designed module for one-stage one-pass RO/EDI/LB filtration system (the thickness of spacer for diluting compartment is 10 mm) . This paper evaluates the changes in voltage-current, resistivity of product water and conductivity of concentrate water, and achieves a proper spacer for diluting compartment. This spacer ensures an even distribution of fluid in diluting compartment, effectively avoids the intermixing between different layers, and realizes the deep removal of ions. It indicates that the resistivity of product water is higher than 17 MΩ?cm, water output per effective membrane area reaches 5.5×10^-5 m³/(cm²?h), and power consumption per ton water is less than 0.7 kW?h when the module is assembled with the modified spacer.