II-VI ternary alloy is a promising material for advanced optoelectronic devices like ultraviolet (UV) photodetectors, light emitting diodes and laser diodes with high efficiency and low fabrication cost. The precise control over the doping element, concentration and crystal quality allows careful tuning of their band structures for novel optoelectronic properties. In this work, we report a prominent two-mode behavior in ternary BeZnO alloy using resonant Raman spectroscopy as a probe. By combining with the excitonic localization and extrinsic Fröhlich interaction, we present a theoretical analysis on the resonant Raman scattering with different Be concentrations and tunable measured temperatures. Frequency blue shifts caused by compressed stress and two-mode phonons (ZnO-like and BeO-like A(LO) phonons) scattering up to third orders revealed in this novel ternary alloy. In addition, alloying induced disorder and electronic states in the forbidden band result in the broadening of line width and the enhancement of LO-phonon oscillator strength. As a result, a high performance BeZnO based photoconductive type UV photodetector was realized, indicating the potential of BeZnO as a promising material for optoelectronic devices.