By use of the Jones vector we deduced the Jones matrix of actual polarizer and light transmission expression, as well as the light attenuation coefficient （LAC） of two polarizers. According to computation, certain error exists when using Malus-Law to calculate polarizer’s light attenuation ratio at high attenuation level. So the deduced LAC expression can be considered as a correction to Malus-Law. We get the maximum light attenuation ratio 2ε2 of two polarizers and light transmission expression about θ2 of three polarizers. Both the expression and experimental data indicate that the maximum light attenuation ratio can reach 2ε4 when［θ1,θ2］=［90°,135°］or［90°,315°］.