Temporal-spatial evolutions of transient plasma in single pulse femtosecond （fs） laser induced microstructure in fused silica were investigated using fs time-resolved pump-probe shadowgraphy. The relation between the spatial distribution of transient electron density and the distribution of fs laser-induced microstructure in fused silica were also studied. In this study， the fs laser was focused by two kinds of microscope objectives with different Numerical Aperture （NA）. The results showed that the transient peak electron density indued by focused fs laser was increased and then decreased as delay time of probe beam increased. When the NA of the microscope objective was 0.45， the spatial position of transient peak electron density induced by fs laser did not moved as delay time increased， which basically kept at the nonlinear focus. The fs laser-induced microstructure in the sample was punctate. When the NA of the microscope objective was 0.3， the spatial position of transient peak electron density induced by fs laser moved from the sample surface to the inside of the sample as delay time increased. The fs laser-induced microstructure in the sample was long strips. In addition， we found that the spatial position of fs laser-induced maximum transient electron density was consistent with the position of laser-induced microstructure when fs laser was focused by the two different microscope objective. Those results imply that fs time-resolved pump-probe shadowgraphy may be used for online monitoring fs laser processing process， which can provide references for directional control of ultrafast laser-induced material microstructure and optimization of machining parameters.