Light emitting diode (LED) has become a mainstream backlight technology for liquid crystal display (LCD) imaging engine of helmet-mounted display.With large emitting angle, a lot of lights from LED can’t be used effectively, causing power waste. The backlight consists of LED array. The secondary optical design can help to improve the luminous efficiency. A double-freeform-surface lens is proposed, which forms a circle spot with a 8mm diameter. The lens is cut to form a rectangle spot. However, the hot spot appears, caused by the total reflection. The lens is then optimized, forming a rectangle spot with high uniformity. 4 lens are combined to become a lens array. In order to eliminate stray lights, the optimization on lens is performed again. The prototype of lens array is developed and the backlight unit with lens array is measured. The measurement results show that compared with the common backlight, the luminance of the backlight with 2 diffusers and lens array increases by 96.4%, the nonuniformity of 23.1% is slightly improved from 23.8%，the viewing angle of half-luminance decreases from 39° to 23°. The designed lens array satisfied the requirements of the LED backlight in LCD imaging engine of helmet-mounted display.