An on-line multi-frequency electrical resistance tomography (mfERT) device with a melt-resistive sensor and noise reduction hardware has been proposed for crystalline phase imaging in high-temperature molten oxide. The melt-resistive sensor consists of eight electrodes made of platinum-rhodium (Pt-20mass%Rh) alloy covered by non-conductive aluminum oxide (Al₂O₃) to prevent an electrical short. The noise reduction hardware has been designed by two approaches: (1) total harmonic distortion (THD) for the robust multiplexer, and (2) a current injection frequency pair: low $f^L$ and high $f^H$, for thermal noise compensation. THD is determined by a percentage evaluation of k-th harmonic distortions of ZnO at $f=0$.1~10,000 Hz. The $f^L$ and $f^H$ are determined by the thermal noise behavior estimation at different temperatures. At $f <1$00 Hz, the THD percentage is relatively high and fluctuates; otherwise, THD dramatically declines, nearly reaching zero. At the determined $f^L\ge$ 10,000 Hz and $f^H\approx$ 1,000,000 Hz, thermal noise is significantly compensated. The on-line mfERT was tested in the experiments of a non-conductive Al₂O₃ rod dipped into conductive molten zinc-borate (60ZnO-40B₂O₃) at 1000~1200 $°$C. As a result, the on-line mfERT is able to reconstruct the Al₂O₃ rod inclusion images in the high-temperature fields with low error, ${\mathsf{\varsigma }}_{f^L, T}$ = 5.99%, at 1000 $°$C, and an average error$⟨{\mathsf{\varsigma }}_{f^L}⟩$ = 9.2%.