An insight into the signal gain in multiple fourwave mixing (MFWM) processes based on a cubic susceptibility medium is theoretically proposed and demonstrated. A dual-pumped fiber optical parametric amplification (FOPA) model is developed with the exact expressions for the signal gain and its 3-dB bandwidth successfully yielded.With these analytical expressions, we show that when pump-1 and pump-2 wavelengths tuned at 1549 and 1551.02 nm, the 3-dB bandwidth of the signal gain can be achieved at about 2253.4 nm theoretically by properly setting the pumped powers. Numerical simulations show that there is a small pinnacle emerging in the signal gain spectrum owing to the degenerate OPA process occurring. The proposed OPA model can find potential applications in all-optical wavelength conversion, optical amplifier, and all-optical sampling.