  In this paper, new determined BVR_cI_c light curves and radial velocities of
V781 Tau are presented. By analyzing the light curves and radial velocities
simultaneously, we found that V781 Tau is a W-subtype medium contact binary
star with a mass ratio of q=2.207+-0.005 and a contact degree of
f=21.6(+-1.0)%. The difference between the two light maxima was explained by a
dark spot on the less massive primary component. The orbital period change of
V781 Tau was also investigated. A secular decrease at a rate of
$-6.01(+-2.28)*10^{-8} d/yr and a cyclic modulation with a period of 44.8+-5.7
yr and an amplitude of 0.0064+-0.0011 day were discovered. The continuous
period decrease may be caused by angular momentum loss due to magnetic stellar
wind. Applegate mechanism failed to explain the cyclic modulation. It is highly
possible that the cyclic oscillation is the result of the light travel time
effect by a third companion.
