The invention discloses an on-line graying point judging device in the titanium white production. The on-line graying point judging device comprises a transmittance detecting unit, an output unit and a control unit, wherein the transmittance detecting unit is connected with a hydrolyzer in the titanium white production and is used for detecting the transmittance of titanium liquid in real time; the output unit is connected with the transmittance detecting unit; and the control unit is connected with the output unit, and receives control signals output from the output unit to control the heating and stirring of the hydrolyzer in the titanium white production. The on-line graying point judging device can accurately judge greying points in the hydrolytic process of the titanium liquid, the prepared titanium pigment has uniform particle size distribution, narrow particle diameter distribution and high pigment performance, and simultaneously, the judgment of the graying points and the relevant control conditions are institutionalized, indexed and automated, thereby providing a new control technique and a new device for the titanium pigment production.