In this paper, we present a novel kind of electron acceleration mechanism when the trapped electrons are in a wake of plasma with adiabatically attenuating density. Accompanied with the increase of phase velocity βp of the wake owing to the plasma density attenuating adiabatically, the trapped electrons synchronized with the wake will get an additional acceleration besides the synchrotron oscillating acceleration just like in an accelerating elevator. The additional energy gain of the trapped electrons is determined by the value of the relativistic factor γpf = ω/ωp(zf) at the exit zf of the plasma wake. Moreover, the acceleration length is not limited by the dephasing deterministically any more, instead, it is mainly determined by the survivability of the wake. In other words, it is dependent upon the wake instability and its involving energy from driving source.