A gantry machining tool with a work piece bench rotation mechanism comprises a base table, a machining table, gantry vertical columns, a cross beam and a machining head. Two first sliding rails are arranged on the base bench; two first sliding chutes which slide along the first sliding rails are formed under the body of the machining table; the vertical columns are arranged on the two sides of the rear of the base table respectively; the top ends of the two vertical columns are mutually connected; two second sliding rails which stretch in an up-down parallel way are arranged at the front end faces of the vertical columns; the cross beam can be slidably arranged on the second sliding rails of the two vertical columns; two third sliding rails which are transversely arranged and stretch in parallel are arranged on the front end face of the cross beam; and the machining head is slidably arranged on the two third sliding rails; and the work piece table rotating mechanism is arranged on the machining table. Consequently, rotation driving is accurate, and the rotation angle of a work piece table is more accurate; the work piece table can more stably rotate; and working efficiency is improved.