Abstract: Objective To refit the existing dental unit water lines with reverse osmosis systems and ultraviolet lamp devices; before and after modification, samples were collected randomly for bacterial culture; investigation of changes of total heterotrophic bacteria in outlet water from dental unit water lines was made. Methods Randomly select five dental units in use, remove the handsets, refit dental unit water lines with reverse osmosis systems at the water intakes; install instantaneous ultraviolet disinfection devices at the water outlets of dental unit water lines in front of the handsets and three-way water syringes. Draw 5 samples randomly before and after modification from mobile interface of each dental unit; take the 50 water samples for bacterial culture; numbers of bacteria were compared. Results Average bacterial colonies of water samples from the handset interfaces of dental units without installation of reverse osmosis systems and ultraviolet devices was 219cfu/ml. Average bacterial colonies of water samples from the handset interfaces of dental units with installation of reverse osmosis systems and ultraviolet devices was 8cfu/ml. Conclusion Dental unit water lines with reverse osmosis systems and uv devices make a remarkable change in water quality, which not only can easily meet the European oral medical water requirements, but also meets domestic Sanity Standard for Drinking Water.