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The new discovered short-period close binary star, XY LMi, was monitoredphotometrically since 2006. It is shown that the light curves are typicalEW-type and show complete eclipses with an eclipse duration of about 80minutes. By analyzing the complete B, V, R, and I light curves with the 2003version of the W-D code, photometric solutions were determined. It isdiscovered that XY LMi is a high fill-out, extreme mass ratio overcontactbinary system with a mass ratio of q=0.148 and a fill-out factor of f=74.1%,suggesting that it is on the late evolutionary stage of late-type tidal-lockedbinary stars. As observed in other overcontact binary stars, evidence for thepresence of two dark spots on both components are given. Based on our 19epoches of eclipse times, it is found that the orbital period of theovercontact binary is decreasing continuously at a rate ofdP/dt=-1.67\times10^{-7}\,days/year, which may be caused by the mass transferfrom the primary to the secondary or/and angular momentum loss via magneticstellar wind. The decrease of the orbital period may result in the increase ofthe fill-out, and finally, it will evolve into a single rapid-rotation starwhen the fluid surface reaching the outer critical Roche Lobe.

High Fill-Out, Extreme Mass Ratio Overcontact Binary Systems. X. The new discovered binary XY Leonis Minoris