  We construct high order symmetric volume-preserving methods for the
relativistic dynamics of a charged particle by the splitting technique with
processing. Via expanding the phase space to include time $t$, we give a more
general construction of volume-preserving methods that can be applied to
systems with time-dependent electromagnetic fields. The newly derived methods
provide numerical solutions with good accuracy and conservative properties over
long time of simulation. Furthermore, because of the use of processing
technique the high order methods are explicit, and cost less than the methods
derived from standard compositions, thus are more efficient. The results are
verified by the numerical experiments. Linear stability analysis of the methods
show that the high order processed method allows larger time step size during
integration.
