A novel structure, called a “freeform surface,” is integrated into a direct type light-emitting diode backlight. By applying the Taguchi method, the performance of this backlight is optimized. The Taguchi experiments are configured in L9(34) orthogonal arrays and are simulated via LightTool analysis software. After that, the influence of the design parameters on the luminance and uniformity are separately evaluated by analysis of variance (ANOVA). Next, the parameters are optimized, and a new backlight structure with desirable performance is designed at last. LightTool simulation shows that this new type of backlight is just 15 mm thick and has 310.3 nits luminance and 83.5% uniformity.