Liquid-level detection is a critical function in fully automated in vitro diagnostic devices, as its accuracy is directly related to reagent aspiration safety and the reliability of test results. Targeting the requirements for precision and stability in contact-based liquid-level detection, this study designs a crystal-oscillator-based capacitive liquid-level detection system. A crystal oscillator is used to generate a highly stable sinusoidal excitation signal that drives a capacitive voltage divider, When the sensing probe contacts the liquid surface, variations in its parasitic capacitance modulate the amplitude of the excitation signal. By combining synchronous band-selective peak detection with a self-stabilizing pulse conversion mechanism, the system achieves highly sensitive and stable detection of the liquid level. Based on an equivalent capacitance model and transfer function analysis, the key component parameters in the system are optimally configured. Simulink simulations verify that the front-end circuit exhibits a response delay of 150 μs and confirm that the self-stabilizing circuit enters a pulse saturation state when the input voltage variation ΔV > 15 mV or the slew rate > 1 V/ms, thereby ensuring system stability under complex operating conditions. Experimental results show that, due to its higher impedance, deionized water produces a lower output pulse amplitude than physiological saline. When the saline concentration exceeds 0.9%, the pulse amplitude exhibits a nonlinear saturation trend with increasing concentration. Within a liquid volume range of 100~5 000 μL, the system achieves a measurement standard deviation ≤ 50 μm and a coefficient of variation (CV) ≤ 0.75%. Over 10 000 consecutive tests, the deviation between two adjacent measurements remains within ±0.15 mm. The slight linear decrease in liquid level is mainly attributed to liquid evaporation and residual adhesion on the probe wall. By operating the crystal oscillator at its resonant point and jointly optimizing the key parameters, the proposed design achieves medical-device-grade detection accuracy and long-term operational reliability, providing a practical and engineering-ready solution for liquid handling in biochemical analyzers, immunoassay analyzers, urinalysis analyzers, and other in vitro diagnostic instruments.