The invention discloses a system and a method for titanium dioxide sizing by the treatment chlorination process. The system comprises a filter press washing device having a titanium dioxide sizing inlet, a filter cake outlet and a filtrate outlet; a settling tank having a filtrate inlet, a flocculation inlet, a supernatant outlet and a bottom sizing outlet; a sizing buffer tank having a buffer tank inlet port, a first film material inlet and a buffer tank outlet connected with the bottom sizing outlet; a filming tank having a first film sizing inlet, a second film sizing inlet and a storage tank outlet. The storage tank inlet is connected with the buffer outlet, and the storage tank outlet is connected with the inlet of the filter press storage tank. The system also comprises the filter press storage tank having the storage tank inlet, the second film material inlet and the storage tank outlet. The storage tank inlet is connected with the buffer tank outlet, and the storage tank outlet is connected with the titanium dioxide sizing inlet. The system not only improves the recovery rate of titanium, but also can achieve the control of titanium dioxide product quality and ensure the continuity of production.