Abstract: The problems, which were existed in the chemical water processing programmed control system such as the devices were aged, the reliability was low, the availability factor of the programmed control was low, the functions were imperfect, the upper level processor of HMI was crashed, the signal communication was blocked, the supply voltage of the field electromagnetic valve was too low，and others, were analyzed. The reformation scheme of the programmed control system and the implement process of the scheme were introduced. The software upgrading and merging, the software and hardware simulation test and the construction and debug in step mode were described. The concrete methods and experiences of programmed control PLC system reformation of chemical water system during unit operation were summarized.

Key words: thermal power plant operation, chemical water processing system, reformation of programmed control system, reformation scheme