Abstract: A femtosecond time-resolved Boxcar pump-probe difference absorption spectrometer is developed. It has three major components: a dispersion compensated optics system of femtosecond laser probe, an argon laser pumped Ti: sapphire femtosecond laser system and a signal detection and analysis system. A two-wavelength tunable Ti: sapphire femtosecond laser system is employed to meet the repuirement of wavelength change of both the pump and the probe. The laser output is tunable from 750 nm to 920 nm and the frequency doubling output is tunable from 375 nm to 460 nm. The minimum time resolution of total Boxer spectrometer is 90 fs and spectral resolution is less than 0.1 nm. The detection sensitivity is about 10^-6～10^-7. Light at 400 nm is used for pump pulses and light at 800 nm is used for probe pulses. This spectrometer has been used to determine the charge separation rate for photosynthesis PSⅡ reaction center D₁-D₂-cyt-b559, a charge separation time of 3.2 ps is obtained.