A nover cavity construction of FEL has beem investigated. Using a hyperboloid-paraboloid grazing incidence telescopes, the cross section of the beam in cavity is expanded approximately by a factor of six, thus the ability of resonator film against laser damage is improved. The effect of the beam expansion the 'parameters of the aspheric mirrors, the length of the cavity and the spot size of the grazing incidence on the optical quality of the system.