This paper describes a novel Smith Purcell FEL experiment with a relativistic electron beam of middle energy and a quasi optical resonator composed of diffraction grating and three mirrors reflector. Coherent radiation with peak power of a few tens of kW at 3 mm waveband is successfully detected from the experimental facility characterized by beam energy of 400～500 keV, pulse length of 70 ns ,pulse beam current of 0.2 kA, and pulse magnetic field of up to 1.2 T.