The invention discloses a large-size direct type LED backlight source and a preparation method. The backlight source structure comprises an edge emission type LED, a metal basal plate and a lower wall casing; the edge emission type LED comprises a conical half-reflecting semi-permeable mirror, an LED chip, packaging resin and an LED lamp holder; a plurality of light reflecting bowls are punched on the metal basal plate and are evenly distributed on the metal basal plate, the lower wall casing is in a frame-shaped structure and is arranged at the lower end of the metal basal plate, a hollow cavity is formed between the lower wall casing and the metal basal plate, the lower surface of the metal basal plate and the upper surface of the lower wall casing are provided with fine structures in the hollow cavity, fine structures at the lower ends of the light reflecting bowls are connected with the fine structure at the upper surface of the lower wall casing, and a liquid absorption core is formed; and support columns are arranged between the adjacent light reflecting bowls, have equal heights with the light reflecting bowls and are connected with the fine structure at the upper surface of the lower wall casing, and a liquid absorption core is formed. The invention provides an LED backlight source structure compromising the performance and the energy consumption under the preconditionof low cost and realizing the lightness and the thinness of a large-size LED and a manufacturing process.