The relations between the precision of the transmittance turning point monitoring system and the thickness error of the monitored layer are analyzed.It is derived that the functions of the optical thickness error of each layer and the error judgement with the turning point by monitor while coating is deposited under different monitoring cases.It has been simulated that the final structure of the deposited coating which monitored by the transmission turning point monitoring approach.The simulated thichness values of each layer after deposition is composed of two parts:One of them is the thickness value correspond to the transmittance extreme searched by one dimensional search,the other is the random thickness error cause by the monitoring approach.The computer simulation is agreement with experimental results.