  We present a timing analysis of the eclipsing post-common envelope binary
(PCEB) DE CVn. Based on new CCD photometric observations and the published
data, we found that the orbital period in DE CVn has a cyclic period
oscillation with an amplitude of $28.08$ s and a period of $11.22$ years plus a
rapid period decrease at a rate of $\dot{P}=-3.35\times10^{-11}ss^{-1}$.
According to the evolutionary theory, secular period decreases in PCEBs arise
from angular momentum losses (AMLs) driven by gravitational radiation (GR) and
magnetic braking (MB). However, the observed orbital decay is too fast to be
produced by AMLs via GR and MB, indicating that there could be other AML
mechanism. We suggest that a circumbinary disk around DE CVn may be responsible
for the additional AML. The disk mass was derived as a
few$\times$$10^{-4}$-$10^{-3}$$M_{\odot}$ , which is in agreement with that
inferred from previous studies in the order of magnitude. The cyclic change is
most likely result of the gravitational perturbation by a circumbinary object
due to the Applegate's mechanism fails to explain such a large period
oscillation. The mass of the potential third body is calculated as
$M_{3}\sin{i'}=0.011(\pm0.003)M_{\odot}$. Supposing the circumbinary companion
and the eclipsing binary is coplanar, its mass would correspond to a giant
planet. This hypothetical giant planet is moving in a circular orbit of radius
$\sim5.75(\pm2.02)$ AU around its host star.
