A junction box includes a cable connecting box, a cover covering the cable connecting box and an o-ring sealing between the cover and the cable connecting box. The cable connecting box includes an insulative block, a plurality of contacting foils retained in the insulative block, a plurality of diodes connecting with two neighborly contacting foils and four walls surrounding around the insulative block. The cover defines a sealing slot for receiving the o-ring. The sealing slot has a bump at a place of the molding joint of the cover to enlarge a deformation of the o-ring for improving a waterproof effect of the junction box.