It is an ultra low-cost solution for liquid detection with infrared LED (light emitting diode) and infra.red sensitive devices. But the day light and movement of detection equipment can easily lead severe noise whichwill cover the desired signal. To decrease the affection of the noise as described, the method is introduced to samplelight from physical environment and mixed light with infrared LED individually in dozens of milliseconds. The sec.ond order difference of the sampled value is used as the reasoning behind the judgment whether the liquid exists ornot. A batch of data is collected to model the change of liquid, and algorithm is established with partition variables.The results show that the desired signal which trigged by liquid could be detected correctly in adverse environment.As the result, this method can be used in real products such as intravenous infusions.