Accurate liquid concentration detection is vital for medical, industrial, and environmental applications, yet conventional methods still face limitations. We propose an ultrasound probe with a self-assembled polymer whispering-gallery-mode (WGM) microcavity, leveraging the photoacoustic effect and a DFB laser tuning system for ultrasensitive quantification. The probe achieves 13.5 Pa sensitivity and 42 MHz bandwidth (-6 dB), ensuring broad applicability in diverse liquids. Overcoming traditional WGM sensor instability, our integrated platform demonstrates a record-low detection limit of 10⁻⁷ g/mL for crystal violet among substrate-free, real-time methods. Unifying microcavity, laser, and electronics into a portable architecture, this system enables high-fidelity, nondestructive liquid analysis with strong anti-interference and endoscopic-deployable sensing in complex environments.