In this paper , new determined BVR _ { c } I _ { 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 \pm 0.005 and a contact degree of f = 21.6 ( \pm 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 ( \pm 2.28 ) \times 10 ^ { -8 } d/yr and a cyclic modulation with a period of 44.8 \pm 5.7 yr and an amplitude of 0.0064 \pm 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 .