The effect of pulse energy on the formation of femtosecond laser plasma filament is studied by the fluorescence spectrum method and the photographic imaging method, and the influence of pulse energy on the length and the starting position of femtosecond laser plasma filament is obtained. The experimental results show that the long plasma filament forms after focusing of lens when the femtosecond laser transmits in the air. As the laser energy is increasing, the starting position of plasma filament moves toward the position of the focusing lens and the plasma filament length increases. The position of the peak intensity of the N2 337 nm fluorescence spectrum is close to the focusing lens, and the whole fluorescence spectrum intensity increases. Compared with the photographic imaging method, the fluorescence spectrum method has high reliability in the plasma filament length measurement. Finally, the influence of pulse energy on the starting position and the length of plasma filament is explained theoretically.