Polyacrylamide as traditional flocculant begins to draw the public awareness because of its non-biodegradable nature which may cause the long-term environmental degradation problems. A new high-efficient flocculant was synthesized by legume starch and acrylamide to satisfy the demand of coalmine wastewater treatment. Grafting acrylamide onto mung bean starch was investigated and the characterizations of elemental analysis, scanning electron microscopy, gel permeation chromatography, Fourier transform infrared, X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry and nuclear magnetic resonance spectrometer were conducted. The characterization results of the new flocculant prove that acrylamide has grafted onto the mung bean starch molecule chains successfully. Experimental results indicate the optimal synthesis parameters of the new flocculant are: mung bean starch 50 g/L (ethanol solvent), acrylamide 100 g/L, ceric ammonium nitrate 12 g/L, gelatinization temperature 70 degrees C and copolymerization temperature 70 degrees C. We also measured and evaluated the performance of the new flocculant on wastewater treatment, and the optimal dosage of the new flocculant is 20 mg/L, with a prominent transmittance of 98.1% and turbidity of 7.82. Experimental results have demonstrated the graft copolymer of acrylamide and mung bean starch owns an outstanding flocculation effect than the traditional polyacrylamide and polyaluminum chloride. The new flocculant has a few features such as environmentally friendly, easy to degrade, fasting sedimentation and low cost, which is especially suitable for the occasions of rigorous environmental requirements and is bound to have broad application prospect for coalmine wastewater treatment.