In water quality measuring methods using ultraviolet spectrum absorption, optical signal detecting techniques are very important. The time-sharing sampling system is adopted to detect background light, reference light and measuring light with one dectector in double light, which can eliminate influences from the drift of source and background. The double wavelength band detection model can decrease the influence of turbidity in water. The techniques make the system more precise and reliable. The standard deviation is less than 1% and the maximum relative error is less than 2% for measurement of standard solutions. The results indicate that the precision and accuracy of the apparatus are good.