We report the discovery of a new transition type Wolf-Rayet (W-R) WN/C star in the Galaxy. According to its coordinates RAJ2000=18:51:39.7,DEJ2000=-05:34:51.1, and the distance (7.11_-1.22_^+1.56^kpc away from Earth) inferred from the second Gaia data release, it is found that W-R 121-16 is located in the Far 3kpc Arm, and it is 3.75kpc away from the Galactic Center. The optical spectra obtained by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope and the 2.16m telescope, both located at the Xinglong Observatory in China, indicate that this is a W-R star of the transitional WN7o/WC subtype. A current stellar mass of about 7.1_-1.1_^+1.7^M_{sun}_, a mass- loss rate of dM/dt=10^-4.97^(_-0.20_^+0.16^)M_{sun}_/yr, a bolometric luminosity of log L/L_{sun}_=4.88_-0.15_^+0.17^, and a stellar temperature of T_*_=47_-5_^+9^kK are derived by fitting the observed spectrum with a specific Potsdam W-R model. The magnitude in V-band varies between 13.95 and 14.14mag, while no period is found. Based on the optical spectra, the time domain data, and the indices of the astrometric solution of the Gaia data, W-R 121-16 is likely a transitional WN/C single star rather than a WN+WC binary.