The invention discloses an automatic horizontal position welding device for circular arc groove welds of an elbow made of a steel plate, comprising a working platform, a trolley positioning, rotating and traveling mechanism and a welding gun positioning mechanism; wherein the working platform is provided with a trapezoidal groove and a V-shaped sizing block; the trolley positioning, rotating and traveling mechanism comprises a central shaft, a connecting rod rotating shaft, a connecting rod, a connecting rod upright column connecting piece, a trolley upright column and a traveling trolley; the welding gun positioning mechanism comprises a rack upright tube, a cross connected pipe fitting, a rack connecting piece, a guide plate, a check block, a guiding axle, a box-type slide block, a spring, a supporting frame, a cross sliding plate, a horizontal slide block and a welding gun chuck, wherein a welding gun is fixed to the welding gun chuck. The device has the advantages of simple structure, easiness for manufacture, convenience for operation, adjustable welding speed, stable running, good welding quality and capability of realization of multi-layered and multiple automatic horizontal position welding for the circular arc groove welds inside and outside the elbow and realization of welding tasks of groove welds of various specifications of elbows with different diameters from small diameter of about 400mm to large diameter of about1420mm.