LX Ser is a deeply eclipsing cataclysmic variable with an orbital period of 0.1584325 d. 62 new eclipse times were determined by our observations and the AAVSO International Data base. Combining all available eclipse times, we analyzed the O-C behavior of LX Ser. We found that the O-C diagram of LXSer shows a sinusoidal oscillation with a period of 22.8 yr and an amplitude of 0.00035 d. Two mechanisms (i.e., the Applegate mechanism and the light-travel time effect) are applied to explain the cyclic modulation. We found that it is difficult to apply the Applegate mechanism to explain the cyclic oscillation in the orbital period. Therefore, the cyclic period change is most likely to be caused by the light-travel time effect due to the presence of a third body. The mass of the tertiary component was determined to be M-3 similar to 7.5M(Jup). We supposed that the tertiary companion is plausibly a giant planet. The stability of the giant planet was checked, and we found that the multiple system is stable.