miscalculation

There are 5 slices in my model with last one fixed ,but when modling dewatering, groundwater level in some nodes fall below its  corresponding elevation  of last slice,it's impossible actually,have i done some incorrect settings?
Any suggestions is welcomed!
Modelnewer  ???

First you should check your boundaries together with your slice elevation. Maybe you find here some reasons for falling dry.
You can supress this effect by setting the constraints and the residual water depth (you will find those settings in the "options-menu").

Zebra

This also might be happening if you are using confined layers. In this case, your are not simulating dewatering, but rather depressurization, since when you choose confined layers, you assume that the layers are fully saturated, regardless of the boundaries you are using.