The invention discloses a horizontal reinforced polysaccharide polymer modification agitating ball mill reactor, and aims to provide a ball mill which is uniform in material agitating, free of dead angle in ball milling, controllable in temperature in ball milling, and convenient to maintain. The horizontal reinforced polysaccharide polymer modification agitating ball mill reactor is mainly structured by comprising an agitating device, a grinding barrel, a constant temperature system and a machine frame, wherein the agitating device and the grinding barrel are horizontally arranged on the machine frame; the agitating device is driven by a motor; a U-shaped frame capable of being swung in a radial direction is arranged on the agitating device; a replaceable plough disc is arranged on the U-shaped frame; the grinding barrel is provided with a double-layer structure which is served as a cooling water sleeve; a coolant inlet and a coolant outlet are arranged on the grinding barrel; a feeding port is arranged in 90 degrees respect to a discharging port; positioning plates are arranged at two ends of the grinding barrel; and when discharging, the discharging port is rotated at 90 degrees to downwards discharge. The horizontal reinforced polysaccharide polymer modification agitating ball mill reactor is a reactor applicable to polysaccharide polymer modification.