Abstract: The main theoretical computational results of SG-1 FEL amplifier, including the selection of primary parameters for SG-1 FEL, the calculation for gain curve (microwave power as a function of wiggler magnetic field, namely magnetic field detuning curve), the main output results with a constant and a tapered wiggler, the contribution from the hight-order waveguide modes, the effects of longitudinal space charge force and the perturbation of the primary parameters of input electron beam on the laser output power is described in this paper. It is shown that the laser output power with a constant wiggler can reach 80MW with an efficiency of about 5%; and for a tapered wiggler, 250MW power and efficiency of 16% may be achieved.