The diagnostic features are the basis to detect and characterize the oil film on water through optical remote sensing. This work shows the results of lab spectral measurements of light diesel oil with thickness ranged 1.0～127 μm. A wavelet transform were performed to the reflectance, and the singularity (388～393 nm) was explored as the indicators of oil film thickness. The results indicate that the reflectance of light diesel oil film is higher than that of water in the range from 350～2 500 nm. There is a reflectance peak near 388 nm when the thickness of oil film is larger than 6 μm, however, no distinguished features could be recognized when oil films were thinner than 6 μm. The wavelet coefficients of the fifth decomposition level by applying Daubechies 4 (db4) mother wavelets proved successful for identifying the singularity of oil film’s reflectance spectra and its accurate position. With the thickness lager than 6 μm, the detail coefficients performed an abrupt change within the range of 388～393 nm, and became more violent while oil films’ thickness increased. This research demonstrated that oil films on water with different thickness could be distinguished based on wavelet detail coefficients, with important implications for detection of oils on water using UV and short wave optical remote sensing.