An on-line oil-in-water monitoring probe based on ultraviolet fluorescence spectrophotometry was introduced. This paper focused on detecting system and control system of the probe. The detecting system included an ultraviolet LED and a photodiode,combined with related components, which acted as signal pickup assembly. The control system contained operational amplifier, low pass filter and analog to digital converter (A/D), which operated the weak signal. Ultraviolet fluorescence analysis technique is a kind of fast,simple,non-contact testing,environment-friendly method,and it could be widely applied in engineering if it is combined with single-chip microcomputer technology.