  Constrains of dark energy (DE) at high redshift from current and mock future
observational data are obtained. It is found that present data give poor
constraints of DE even beyond redshift z=0.4, and mock future 2298 type Ia
supernove data only give a little improvement of the constraints. We analyze in
detail why constraints of DE decrease rapidly with the increasing of redshift.
Then we try to improve the constraints of DE at high redshift. It is shown that
the most efficient way is to improve the error of observations.
