Varied with some optical parameters, such as center wavelength, power, the differential reflectance spectroscopy of unintentionally doped high-purity n-type gallium arsenide were studied by femtosecond laser pump probe technique. These time-resolved differential reflective spectroscopy further analyzed the dynamics of photo-induced carrier of gallium arsenide at room temperature. Firstly, if the pump and the probe power were stabled at 100 mW and 10 mW respectively, the peak differential reflectivity increases with the red shift of center wavelength, and the signal-to-noise ratio increases as well. Secondly, Based on the fitting experimental data of varying pump power and the theoretical model, it is found that there is a linear correlation between the pump power and the differential reflectance in a certain range, through which the saturated carrier concentration of this gallium arsenide sample was calculated as (3.590 1±0.310 3)×1017 cm－3. Thirdly, the dynamics process of photo-induced carriers is divided into three terms: the photo-excitation process (804±67 fs), the initial scattering process (134 ~268 fs), the recombination process with 1 picosecond and 3~6 ps. Last but not least, it seems that the differential reflectance has no significant dependence on the probe power, but the signal-to-noise ratio of the differential reflectance spectrum is correlated with the probe power. In a word, this work not only investigates the ultrafast dynamics of unintentionally doped high purity n-type gallium arsenide, which provides a reference for other materials, but also offers the optimal experimental parameters of pump-probe spectroscopy.