Combining with our newest CCD times of light minimum of EM Cygni, all 45 available times of light minimum, including 7 data with large scatters, were compiled and an updated $O-C$ analysis was made. The best-fit for the $O-C$ diagram of EM Cygni was a quadratic-plus-sinusoidal fit. The secular orbital period decrease rate ($-$2.5 $\pm$ 0.3) $\times$ 10$^{-11}\ $s s$^{-1}$ means that a magnetic braking effect in EM Cygni with a mass-loss rate caused by stellar wind, 2.3 $\times$ 10$^{-10}\ M_{\odot}\ $yr$^{-1}$, is needed for explaining the observed orbital period decrease. Moreover, for explaining the significant cyclical period change with a period of $\sim\ $17.74 $\pm$ 0.01 yr, shown in the $O-C$ diagram, magnetic activity cycles and a light travel-time effect were considered in detail. The $O-C$ diagram of EM Cygni cannot totally rule out the possibility of multi-periodic modulation due to gaps present after 25000 cycles. Based on the hypothesis of a K-type third star in the literature, a light trave-time effect may be a more plausible explanation. However, the low orbital inclination of the third body ($\sim\ $7$^\circ\!\!\!.$4) suggests that the hypothetic K-type third star may be captured by EM Cygni. But, by assuming the spectral contamination from a block of circumbinary material instead of a K-type third star, the third star may be a brown dwarf in the case of a coplanar orbit with a parent binary.