We design an atom-waveguide system to achieve nonreciprocal frequency conversion. Single photon frequency conversion is achieved through inelastic scattering mediated by the V-type giant atom with one transition coupled to a one-dimensional (1D) coupled-resonator waveguide (CRW) and the other transition coherently driven by a classical field. To quantitatively characterize the frequency-conversion efficiency, we define scattering flows by incorporating the corresponding group velocities. The reciprocity and nonreciprocity of the system can be characterized by the conversion flow contrast. It is found that the single-photon frequency-conversion flow is closely related to the size of the giant atom and the phase difference between the coupling strengths at the two atom CRW coupling sites. Under reciprocal conditions, the maximum conversion flow is strictly bounded by 1/2, regardless of how the giant atom size and the phase difference are tuned. When reciprocity is broken, tuning the coupling phase at a fixed giant atom size significantly enhances the maximum conversion flow, which can even reach unity. Meanwhile, the conversion-flow contrast reaches its extremal values, corresponding to perfect single-photon nonreciprocity.