The filamentation of a focused femtosecond pulse in fused silica was systemically investigated both experimentally and numerically. We observed the different filamentation of a laser pulse under different external focusing conditions and input energy experimentally. For instance, a focused femtosecond laser pulse with an energy of 3.3 \ensuremath{\mu}J induced a long electron plasma filament for an effective numerical aperture (NA) of 0.01 of focusing objective, an electron plasma filament with two-foci structure for 0.04 effective NA, and a short filament for effective NA of 0.09. In theory, the numerical results of the extended nonlinear Schr\"odinger (NLS) equation agreed with experimental results under different focusing conditions and energy. Moreover, under different focusing conditions we found the filamentation is connected with different temporal reshaping behaviors, and the temporal compression of the reshaped laser pulse in propagation is responsible for the two-foci structure of an electron plasma filament.