Transmitted second harmonic generation (SHG) study was made on Stilbazium salt Langmuir-Blodgett monolayer with different polarization combinations (s→p,p→p and 45°→s). The experimental data fitted in well with the transmitted SHG theoretical expressions.-And when the angle of incidence increased the theoretical data deviated from the experiments. This phenomenon may be due to the neglecting of the multiple reflection of fundamental light and harmonic light in the glass substrate on which LB monolayers were deposited. We also investigated the reasons for such deviation and gave a corrected explanation. It is found that multiple reflection in glass substrate should be considered when transmitted SHG was made with s-polarization and fixing detectors were used.