A modified phase demodulation method for measuring nanosecond fluorescence lifetime is introduced in this paper.The method is based on phase modulation technique and Fourier analysis of both the excitation and the emission waveforms.Excitation light is modulated by an arbitrary periodic wave.Emission of the sample follows the excitation light

is also modulated and with a delay because of the fluorescence lifetime.In the modified method

the first harmonics of the Fourier series are figured out from the excitation and the emission waveforms.The lifetime can be obtained by the relationship same as in the ordinary phase modulation method from the phase difference between the first harmonics.A test system

composed of an oscilloscope

a spectroscope and an excitation light source

was designed and developed.An LED light source modulated by a crystal oscillator is built at an acceptable cost for the system.The emission and excitation are detected by a spectroscope

converted to electrical signal by a photomultiplier tube

digitalized by a digital oscilloscope

and then transmitted to a computer.Fluorescence lifetime is then obtained by Fourier analysis of the emission and excitation waveforms.The relationship between the optimized modulation frequency and the range of the measured lifetimes is discussed.The lifetime of cresyl violet ethanol solution(0.0033 mol/L) measured with this method is(4.03±0.87) ns which is comparable with that by a commercial equipment.