The present invention provides a 3D image signal controlling method. The method comprises inputting a left eye image into the first liquid crystal layer, turning off the backlight module and switching gray levels of the second liquid crystal layer to a minimum gray level during (4N−3)th time interval, wherein N is natural number; stopping inputting the left eye image, turning on the backlight module and remaining the gray levels of the second liquid crystal layer as the minimum gray level during (4N−2)th time interval; inputting a right eye image into the first liquid crystal layer, turning off the backlight module and switching the gray levels of the second liquid crystal layer as a maximum gray level during (4N−1)th time interval; and stopping inputting the right eye image, turning on the backlight module and remaining the gray levels of the second liquid crystal layer as the maximum gray level during (4N)th time interval.