Although numerous efforts have been dedicated toward developing fiber optic distributed acoustic sensing (DAS) system with high performances, challenges still remain in achieving DAS under high-temperature condition. This study thoroughly evaluates the influential mechanism of high temperature ( $\sim 300~^{\circ }$ C) on the $\emptyset $ -OTDR-based DAS system. Experimental results demonstrate that phase shifts of the interferometric signal are linearly proportional to the temperature variations. Temperature-induced thermal strain shows as a wide-spectrum low-frequency signal, and the center frequency component approaches zero. Since the generated thermal strain can lead to the signal-to-noise ratio (SNR) deterioration and the false alarms of DAS system, in this study, variational mode decomposition (VMD) is first proposed to separate the temperature-induced thermal strain from the effective acoustic signal, and the SNR of DAS system for detecting audio acoustic signal in high temperature has been improved to 21.2 dB. Furthermore, dual-parameter measurement of relative temperature and acoustic sensing can also be realized using VMD method. When compared with the previously reported dual-parameter measurement methods based on the combination of DAS/DTS, single back Rayleigh signal is acquired, and back Raman signal is no longer needed, which largely reduces the system volume and demodulation complexity.