AB S T R AC TNGC7789 is an intermediate-age open cluster with an age that is similar to the mean age of contact binary stars. V12 is a bright UMa-type binary star with an orbital period of 0.397day. The first complete light curves of V12 in V, R, and I bands are presented and analyzed with the W-D (Wilson and Devinney) method. The results shows that V12 is an intermediate-contact binary ( f = 43(±2.2)%) with a mass ratio of 3.848, and i t is a W-type contact binary where the less massive component is slightly hotter than the more massive one. The asymmetry of the light curves are explained by the presence of a dark spot on the more massive component. The derived orbital inclination ( i = 83.6 ) indicates that i t is a total eclipsing binary, which suggests that the determined parameters are reliable. The orbital period may show a long-term increase at a rate of P = +2.48(±0.17) × 10 days/year that reveals a rapid mass transfer from the less massive component to the more massive one. However, more observations are needed to confirm this conclusion. The presence of an intermediate-contact binary in an intermediate-age open cluster may suggest that some contact binaries have a very short pre-contact timescale. The presence of a third body or/and stellar collision may help to shorten the pre-contact evolution. Key laboratory of the structure and evolution celestial bodies, Chinese Academy of Sciences,