The invention provides a straw briquetting mechanism with a vertical shaft, a flat die and a straight roller, and belongs to the technical field of agricultural machinery. The straw briquetting mechanism mainly comprises a base, a transmission system, a collection hopper, a support, the main shaft, the flat die, a pressing roller frame, the pressing roller and a sleeve. The main shaft is vertically installed at the center position of the base and the center position of the support. The flat die is horizontally installed on the support. A pressing roller frame sleeve is vertically installed on the main shaft and fixed on the support. The pressing roller is horizontally installed at the radial shaft end of a pressing roller frame sleeve. The flat die is sleeved with the sleeve. The collection hopper is installed at the lower end of the flat die and fixed to the support. The flat die is provided with a plurality of square through holes within the circumference, and the center line of each hole coincides with the rotation center line of each pressing roller tooth, so that it is ensured that straws can be successfully pressed into the holes of the die. The pressing roller is in a straight gear shape, rotates along with the shaft under the driving of the pressing roller frame, and meanwhile rotates around the axis of the pressing roller to press the straws into the holes of the die continuously, so that straw blocks with the regular shape and high intensity are formed, and the production rate and the ratio of briquetting are high. The straw briquetting mechanism is simple and compact in structure, the holes of the flat die are not prone to being blocked, and working continuity is good.