The characteristics of plasmas play an important role in femtosecond laser filament-based applications. Spectroscopic analysis is used to experimentally investigate the plasma density and its temperature of the air filament under different pulse repetition rates. In our experiments, the measured average plasma density of the filament is $1.54\times {10}^{17}{\mathrm{cm}}^{-3}$ and the temperature of the plasma is about 5100 K under 100 Hz pulse repetition rate. The plasma density decreases to $1.43\times {10}^{17}{\mathrm{cm}}^{-3}$ and the temperature increases to 6230 K as the pulse repetition rate increases to 1000 Hz. The experimental observation agrees with the numerical simulation by solving the nonlinear Schrödinger equations with repetition rate related “low density hole” correction.