a screening method of a phosphor - based optical film used in a backlight module and a backlight module are proposed . the screening method includes steps of : a ) dividing an internal surface of the backlight module without a phosphor - based optical film into multiple measurement zones , and gaining a transmittance spectrum of each of the measurement zones ; b ) gaining a chromatic value of each of the measurement zones matching the phosphor - based optical film ; c ) checking the chromatic values gained in b ) to be within a range of standard chroma ; the process of screening being finished if all of the chromatic values are in the range ; matching at least one measurement zone with a new phosphor - based optical film if the chromatic value of the at least one of the measurement zones is not within the range , and returning to b ). the present backlight module has higher saturation and penetration and better color uniformity .