To guarantee the consistency and reliability of the fiber drop analysis technique, it is important to understand the propagation of light inside the liquid drop in detail. Three experimental systems in different directions were designed and fabricated. Pure water was chosen as the experimental sample. The light fields of refracted light in three directions from the liquid drops were monitored. Through analyzing the distribution of the refracted light and the experimental law of output light-field from fibers, the transmission traces and the propagation law of the light inside the liquid drops were proposed. The forming process of peaks of fiber fingerprint drop traces (FFDT) was explained. The experimental results show that different peaks of FFDT correspond with different transmission modes of light inside the liquid drop.