This work investigated the separation process and contents of ortho toluene diamine (O-TDA), metra toluene diamine (B-TDA) of by-product tar of amine light gasification. We discussed the effect of the pressure, temperature, speed, dripping speed on O-TDA, B-TDA contents in tar coupling with the falling film evaporation technology. The contents of TDA in the tar were quantitative identified by orthogonal experiment optimization. The results show that the optimal separation conditions are at temperature of 200℃, 15 kPa pressure, rotating speed of 30 r/min, and the feed rate of 1.5 r/min, then the separating efficiency is best with a relative standard deviation of 1.1%?1.5%.