The invention provides a horizontal sanding apparatus for an electroplated diamond wire saw. The apparatus comprises an electroplating tank, the electroplating tank contains a plating solution, an anode is placed in the plating solution, a sand barrel is horizontally arranged at the bottom of the electroplating tank, the sand barrel is completely immersed in the plating solution, a metal wire is driven to be immersed in the plating solution in the plating tank by a pay-off wheel, goes through the sand barrel from the left to the right, and then departs from the electroplating tank through a take-up wheel, the sand barrel is connected with a driving device capable of driving the sand barrel to rotate with the metal wire as a rotating shaft, a sand inlet for adding diamond powder is arrangedin the top wall surface of the sand barrel, the left end, positioned at a metal wire inlet, of the sand barrel is provided with an impregnation solution opening to communicate with an external plating solution, and the right end, positioned at a metal wire outlet, of the sand barrel is provided with a sand blocking groove to prevent the diamond powder from flowing out with the metal wire. The sanding apparatus has the advantages of uniformity in sanding, high content of sands in the sand barrel, compactness in sanding, no shedding, and reduction of the use amount of the diamond powder.