With the increasing availability of high output power erbium-doped fiber amplifier and the development of highly nonlinear fibers, more gain saturation phenomena are observed in fiber optical parametric amplifiers (OPAs), which are expected to be avoided. A practicable method is proposed to avoid the gain saturation phenomenon, that is optimizing fiber length according to other parameters of fiber OPA, such as nonlinear coefficient, input pump power and input signal power. The concept of optimum fiber length is introduced, namely the minimum fiber length needed for the maximum signal gain or signal output power with other parameters fixed. The optimum fiber length is of great benefit to boost the gain bandwidth and the conversion bandwidth. By solving the nonlinear coupled equations that govern the fiber parametric amplifying process with numerical integration as well as adopting controlled variable method, the optimum fiber length as a function of nonlinear coefficient, input pump power and input signal power is profoundly studied. Finally, coefficients are figured out with least square method, and a pithy and useful analytical expression for the optimum fiber length is presented. Compared with existing experimental result, this analytical expression could be soundly used to optimize fiber length for fiber OPAs.