The invention relates to the technical field of machining equipment, aims to solve the problems of out-of-tolerance error, low machining precision and low machining efficiency due to the fact that workpieces are required to be mounted and dismounted repeatedly during machining in the prior art and solve the problem of difficulty in clamping, positioning and alignment of workpieces, particularly casing type workpieces, and provides a positioning and clamping device for machining of an engineering machinery casing. By the aid of the device, the purpose of changing the machining positions of theworkpiece on the premise that the workpiece is not required to be mounted and dismounted is achieved; a first positioning mechanism and a second positioning mechanism are arranged, so that the workpiece can be positioned stably when located in the different machining positions, the machining precision and the machining efficiency for the workpiece are improved, and the device is convenient to mount and dismount and is easy to operate.