A waterproof electric plug is provided with a housing including two forward prongs, an open rear end, a closed groove on the rear end, and a closed trough on the rear end wherein length of the trough is greater than that of the groove; a transformer in the housing and electrically connected to the prongs; a cap including a closed projection on a forward end, and a closed protrusion on the forward end wherein length of the protrusion is greater than that of the projection; an electrical socket disposed through the cap and electrically connected to the transformer; and adhesive disposed in the groove. The projection is complimentarily disposed in the groove to press the adhesive for forming a first waterproof structure. The protrusion is complimentarily disposed in the trough to form a second waterproof structure.