In this study, light distribution function is applied to the LED direct backlight unit (DBLU) with the Gaussian optical field distribution fitting polynomial as the objective function, using a nonlinear least squares method. The distribution function of the LED light provides a light distribution simulation algorithm, for the reflection of the light source and light box, which accurately simulates the LED DBLU authentication to obtain light distribution function calculation results. A simple LED module is designed for authentication. Comparison with the experiment, simulation error of a single light source can be less than 0.56% and the overall simulation error is less than 0.81%. Obvious, it is a valid simulation method for a LED module.