Overview: When using crude oil for producing qualified oil and gas production, we need to separate the water from the crude oil which is called oil sewage. The oil sewage contains a certain amount of oil and other impurities. The oil sewage is usually reinjected back under the ground after multiple processes. To avoid the excess oil of reinjection water affecting the quality of underground water source, it is important to ensure that the oil concentration is lower than requirement. Oil concentration measurement methods include infrared spectrophotometry, nephelometry, ultraviolet spectrophotometry and ultraviolet fluorescence. The ultraviolet fluorescence method has the advantages of real-time and high precision. The oil concentration of purified reinjection water is usually lower than 10 mg/L, so the ultraviolet fluorescence method is more suitable in this situation. To accurately measure the concentration of reinjection water based on ultraviolet fluorescence method, high transmission power of light emitting diode (LED) is required, so that the fluorescence of aromatics in oil is sufficient to be detected. In early experiments, the oil concentration is measured by square-wave dimming method (duty ratio equal to 50%). The chip temperature of LED will be higher than requirement because of the high transmission power of LED, thereby, LED luminous efficiency will be drastically reduced, and LED devices will be even damaged. To solve this problem, this paper proposed the pulse modulation dimmer method (duty ratio less than 50%) to reduce the LED average power and increase instantaneous power, which could ensure high signal-to-noise ratio (SNR). Pulse modulation dimming method makes LED produce higher transient light intensity and shortens the amount of conduction time in unit time. Two noise analysis models for pulse modulation and square-wave modulation dimming method were built. The discussion focused on wideband limiting in square-wave dimming method and the influence of noise aliasing in pulse modulation dimmer method. Signal-to-noise ratio of measurement results for two methods were compared under several condition. The relationship among SNR, light source power and duty ratio was derived. The optimum choices of current amplitude and duty ratio for pulse modulation dimming method were proposed under the condition of the same signal-to-noise ratio. Results show that the light source power for pulse modulation dimming method is less than 21% of that for square-wave modulation dimming method under the condition of the same signal-to-noise ratio. Finally, signal demodulation models of two dimming methods were simulated by MATLAB software. The simulated relationship of SNR for two methods is consistent with the theoretical analysis results.