A clamp for crop stem bending test comprises a clamp holder (3) and a fixed chuck with an annular clamping cavity. The fixed chuck comprises a semicircular upper chuck (1) and a lower chuck (2), the bottom of the lower chuck (2) is fixedly connected with the clamp holder (3), and the upper chuck (1) is detachably connected with the lower chuck (2). A pushable arc-shaped inner chuck (4) is arranged in the annular clamping cavity of the clamp, the top of the inner chuck is vertically, upwardly and fixedly connected with a pitch adjusting screw (9), the pitch adjusting screw (9) is connected with a pitch adjusting nut (10) outside the fixed chuck through a hole at the top of the fixed chuck in a threaded manner, and a clamp groove (102) is arranged at the bottom of the pitch adjusting nut (10) and slidably clamped with a clamp ring (103) corresponding to the upper chuck (1). Straw positions with different diameters are fastened by the pushable arc-shaped inner chuck, the contact area of the inner surface of the clamp and surfaces of straws is increased, the pulling resistance of the clamp is improved, and the problem that ends of the straws are squashed is effectively reduced.