The influence of concentration (mass fraction, C1) and concentration of acid radical ion (mass fraction, C2) of TFT - LCD developer on Halftone THK is researched, and the way to improve THK fluctuations is put forward. Firstly, the production conditions of lithography process are fixed, then the influence of THK with C1 and C2 is respectively tested using the single factor experimental method. Studies show that the changes of C1 and C2 will affect the developer’s development ability and lead to the changes of THK. THK with C1 and C2 are both linear negative correlation. Secondly, according to the influence degree of C1 and C2 changes on THK, the formula corresponding relations between the two is established. The C1 is adjusted through the system and then the C2 fluctuations are improved. Lastly, the development ability and the THK fluctuations are stable through putting the formula into the system which make channel related incidence reduced to below 0.03% from more than 0.20%.