An optical liquid drop sensor detects the light signal passing through the liquid drop during drop growth. A drop head is used to form the drop. A 90° reserve cone angle at its lower end is specially designed based on surface theory, to ensure that the liquid can fully wet the end surface of the drop head. How to transmit the light signal with low loss in both intensity and spectrum is a key technology. Optical fiber transmission is a simple and convenient method if the transmission characteristic of the fiber match the spectral width when studying a specific liquid. The coupled light signal is related to the penetration depth of fibers into the liquid drop. So a fiber height adjuster is desired. A geometrical microoptical path is favorable for studying the absorption spectrum of the multiwavelength light signal passing through the liquid. Spherical lenses inside the drop head are employed as the light injector and collector. They are positioned symmetrically so that the drop profile is not influenced by different contact surfaces. Experimental graphs of some samples are presented.