Abstract: In this work,spectrophotometry was used to characterize the grey point of hydrolysis during titanium dioxide preparation via sulphate process.Employing the self-developed on-line grey point determination instrument,the effects of scanning wavelength at a certain range and dilution ratio on the transmittance at gray point were studied for the hydrolysis process within adding external seeds,and the optimum scanning wavelength and dilution ratio were determined.Furthermore,the influences of two methods of grey point judgement,the on-line determination by the instrument and artificial observation,on the control level and process capability of hydrolysis process were compared.The results show that the control level and process ability of hydrolysis process could be improved significantly by using the self-developed grey point detector instead of artificial observation.