The invention discloses an automatic horizontal boring and milling machine which is mainly composed of a machine body (1), a lower sliding seat (2), an upper sliding seat (3), a workbench (4), a spindle box (5), a vertical post (6), a boring shaft (7), a feeding servo motor (8), a guide rail and an electric system, wherein the machine body (1) is provided with two guide rails, namely, a guide rail A (20) and a guide rail B (21), both of which have the same horizontal height and are in parallel arrangement; a workbench clamping mechanism is also arranged between the upper sliding seat (3) and the workbench (4); the workbench clamping mechanism is specifically composed of a hydraulic cylinder (24), a piston (25), a disc spring (26), a piston rod (27) and a round gasket (28). The automatic horizontal boring and milling machine has a simplified structure, low fault rate, high machining precision, clamping reliability of the workbench and expectable huge economic value and social value.