An illuminating device includes a light source module for emitting light and an optical lens for adjusting the light. The light source module includes a reflecting unit and LEDs. The reflecting unit includes strip-shaped grooves each extending along a first direction. The LEDs are mounted on the reflecting unit in the grooves. The optical lens includes an array of lens units each including a main body, a light diverging portion and a light converging portion. The light diverging portion is for expanding a light field of the LEDs along the first direction. The light converging portion is for compressing the light field along a second direction. The reflecting unit is for further compressing the light field along the second direction.