An ultra-thin direct-down LED panel model with tapered optical elements was proposed to cover the shortcomings of large thickness and light density of the direct-down LED panel light. The diffuse reflection backboard and the reflective cone structure in this model can increase the LED light mixing distance and improve light uniformity. Using the idea of dividing unit,a hexagonal light source is used as a unit to study in Taguchi method by designing orthogonal experimental group. With the Tracepro software simulation,the five factors influencing light uniformity and efficiency are taken into consideration,including the radius on upper and lower surfaces of the truncated cone element,thickness,height and LED spacing of the hexagonal light source. Apply ANOVA theory to analyze the impact of those factors on the quality,then optimize the structure parameters of the light. And ultimately,a direct-down LED panel light with the thickness of only 15 mm,the light uniformity up to 95.3% and the light efficiency up to 92.97%,are designed.