The frequency conversion performance of a large scale rapid growth KDP crystal is studied. Using the orthogonal polarization interferometry (OPI) technique, the spatial refractive-index non-uniformities which prohibit the attainment of phase matching at all locations in the crystal is measured. Then third harmonic generation (THG) experiments with the large scale rapid growth KDP doubler on the prototype of SG-III laser facility to estimate the influence of refractive-index non-uniformities on conversion efficiency and 3ω beam near-field are conducted. According to the experimental results, this large scale rapid growth KDP crystal meets the basic requirements. But for specific applications, other aspects such as the stimulated Raman scattering gain coefficient and laser-induced damage threshold should be further considered.