Abstract: Stable cavitation, characterized by the periodic oscillation of microbubbles in water under low acoustic pressure, contrasts with transient cavitation due to its stable and controllable temperature increase, presenting more promising applications. This paper describes the construction of a monitoring platform for the stable cavitation threshold, employing a passive cavitation detection method. The method monitors stable cavitation activity in focused acoustic fields in water with varying oxygen contents, using the emergence of subharmonics as an indicator of stable cavitation. Combining narrow-band mechanical filtering and multiple averaging techniques, the acoustic signals received by high-sensitivity hydrophones are filtered and noise-reduced. Spectral analysis of these signals explores the relationship between oxygen content and the appearance of subharmonics under ultrasonic influence. The results demonstrate that multiple averaging effectively reduces noise interference and improves signal-to-noise ratio, enhancing the detection of weak signals. Additionally, increasing oxygen content lowers the stable cavitation threshold, aligning with theoretical expectations.