The compression of electron pulses in the temporal domain is a core technique to improve the temporal resolution of ultrafast diagnosis instruments,such as streak cameras and ultrafast electron diffraction systems. In this paper, a time focusing technique is adopted to potentially improve the physical temporal resolutions of streak cameras and ultrafast electron diffraction systems. This method uses a time-dependent acceleration field to greatly compensate the temporal dispersion between photocathode and anode to accelerate the slow electrons and decelerate the fast electrons,relatively. As a result, the temperal dispersion due to an initial energy spread can be compensated to a large extent at the output of time focusing region. Tracing and simulating a large number of photoelectrons through Monte-Carlo and finite difference methods shows that the electron pulse with a 300 fs can be compressed to 50 fs, which lays a powerful foundation for developing the streak cameras and ultrafast e-lectron diffraction systems with better than 100 fs temporal resolution.