The invention provides a device and machining method for fixing a pressure box outside a circular pipe and belongs to the field of geotechnical engineering. The device for fixing the pressure box outside the circular pipe comprises a connection pipe, an arc-shaped plate, a fixing groove and a baffle. The machining method comprises the steps that (a) a first mounting groove and a second mounting groove are formed by cutting in sequence; (b) the arc-shaped plate is turned over and then welded in the second mounting groove; (c) a fixing pipe and a notch are cut, and one end of the fixing pipe isblocked; (d) the end, provided with an opening, of the fixing pipe faces outwards and is partially exposed out of the connection pipe; (e) a pressure box binding post is placed in a space formed by the arc-shaped plate through the notch; (f) the baffle is connected to the fixing pipe in a welded mode; and (g) a pressure box lead wire is led out by the pressure box binding post to be connected to ameasuring instrument. According to the device and machining method for fixing the pressure box outside the circular pipe, the pressure box can be fixedly mounted outside the circular pipe. The fixingdevice does not damage the integrity and airtightness of the original pipe, it is important that the liquid flow in the pipe is not affected, and the external size of the circular pipe is not increased excessively.