A new scheme for all-optical AND gate and NOR gate based on simultaneous four-wave mixing (FWM) and cross-gain modulation (XGM) in a semiconductor fiber ring laser (SFRL) is proposed, and a comprehensive broad-band dynamic model of this kind of all-optical logic gates is presented. By numerical simulation, the effects of the input signal peak power and the coupling of the couplers in SFRL on the output performances of the logic gates are analyzed.