Abstract: The accurately determination of coal and rock impact tendency was importance to rock burst prevention and control.Based on research results of coal seam impact tendency at home and abroad，to the problems that the actual coal seam impact tendency could not be reflected by impact tendency index at present，then an effectively impact energy rate index was put forward，which could be fully reflected energy accumulation，dissipation and releasing process and time effect and so on，then determination criterion was put forward.Some relativity appeared between normal index，which is about effectively impact energy rate，dynamic broken time，elastic energy index, impact energy index and uniaxial compressive strength.The field test showed that impact tendency determined by the index was reasonable.