Abstract: The generalized theory of double-stage electrostatic reflection system for time-domain compressing electron packet is presented. In analog to the multilayer dielectric optical chirp mirror for compressing optical pulse, the system is renamed as tunable electrostatic chirp mirror, of which the tunability of chirp properties is analyzed detailedly. The essential difference between the two chirp properties lies in the dispersive dependence of electrons time of flight on its initial energy: the electrons with higher initial axial energy have longer time of flight for positive chirp, while it is not the case for negative chirp. The time of flight dispersive dependence constitutes the mechanism of electron packet compression for each mode. And an example illustration is given.