To support the need for the design of sampling module in the full-automatic enzyme immunoassay instrument, this paper completed the design and verification of liquid level detector based on capacitive sensor. Using the capacitance effect existing between the sample needle and the ground (the machine frame), we take the sample needle and the ground as the capacitor poles. By means of the RC oscillation circuit, we convert the change of capacitance to frequency for a measure, in order to achieve the level detection. The stepping motor controls the sample arm with the modular design, and C8051F330 is chosen as main control chip in this system. Through the experimental analysis, we proved the reliability of the detection of the sample level based on the capacitance liquid level detection principle. This study provided a theoretical basis to the level parameters of the full-automatic enzyme immunoassay instrument.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.