We present a timing analysis of the eclipsing post-common envelope binary (PCEB) DE CVn. Based on new CCD photometric observations and 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 <(P)over dot> = -3.35 x 10(-1)(1)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 another 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 x10(-4)-10(-3) M-circle dot, which is in agreement with that inferred from previous studies in the order of magnitude. The cyclic change is most likely the result of the gravitational perturbation by a circumbinary object due to the Applegate's mechanism failing to explain such a large period oscillation. The mass of the potential third body is calculated as M-3 sin i' = 0.011(+/- 0.003) M-circle dot. Supposing the circumbinary companion and the eclipsing binary are coplanar, its mass would correspond to a giant planet. This hypothetical giant planet is moving in a circular orbit of a radius of similar to 5.75(+/- 2.02) au around its host star.