In situ Raman spectra of Al3BC3 have been measured at elevated temperatures of up to 1000 °C and high pressures of up to 32 GPa. It is shown that there are no abnormal phonon shifts when the measurement conditions were up to 1000 °C or up to 27 GPa, which indicates a good structural stability of Al3BC3 at high temperatures and pressures. The Raman active phonon modes were assigned comprehensively to the corresponding vibration modes by first-principles calculations. Interestingly, an abnormal softening of the E1g(559 cm–1) and E2g(578 cm–1) phonon modes were observed when the pressure was higher than 27 GPa. Correlating the results with previous theoretical predictions of polymorphism in Al3BC3, the mode softening at higher pressures might originate from the structural transformation (from hexagonal to tetragonal symmetry) involving changes of Al and C coordination numbers. In addition, the phonon anharmonicity has been investigated by studying the temperature and pressure dependence of the Raman peak shifts and broadenings. Finally, the present work has highlighted that in situ Raman spectrometry study is a sensitive method for investigating the structural stability and phonon anharmonicity of complex ceramics.