The invention provides an adjustable sieve mesh mechanism of a feed hopper of a threshing machine and belongs to agricultural machinery. A V-shaped movable sieve piece with movable sieve piece holes is arranged on the lower portion of the bottom side of a V-shaped fixed sieve piece with fixed sieve piece holes in a containing and covering and transversely-adjusting moving mode, a pull rod is fixedly arranged on one side of the V-shaped movable sieve piece, a lead screw is installed on the outer portion of the bottom side of the feed hopper in an axially and radially positioned and circumferentially rotating mode, a nut plate is arranged on the lead screw in an axially moving mode, and the nut plate is fixedly connected with the pull rod. A handle is fixedly arranged at the end of the lead screw, and arc chamfers are arranged on the peripheries of the upper ends of the walls of the fixed sieve piece holes in the V-shaped fixed sieve piece. According to the mechanism, the size of the leakage flux area of the fixed sieve piece holes in the V-shaped fixed sieve piece is adjusted, and the requirements for discharging operation of different snow powder amounts and dropping grain amounts are met. The adjustable sieve mesh mechanism is novel, reasonable and simple in structure, easy and convenient to adjust, high in adaptability, good in use performance, high in working quality and low in grain breakage rate.