A short cavity Er/Yb co-doped fiber laser of high output power and high slope efficiency is demonstrated. Its resonant cavity is composed of a section of single mode Er/Yb co-doped fiber and two fiber Bragg gratings with the same reflection wavelength. A fiber grating is used for output of resonant cavity with 60% reflectivity. Its 3 dB-bandwidth is measured as 0.16 nm. The other fiber grating is used for high broadband reflector and input of pump light with 99% reflectivity. Its 3 dB-bandwidth is measured as 1.02 nm. This fiber laser is pumped by a 980 nm semiconductor laser. Meanwhile, the Er/Yb co-doped fiber with different lengths is measured by different pump power, and the optimized length of the fiber is only 13 cm. A short cavity fiber grating laser is done by using 13 cm Er/Yb co-doped fiber as gain medium. Its maximum output power is 11 mW and its power stability is better than ±0.01 dB. The laser has a threshold of 35 mW and a slope efficiency of 15.3%. Its 1552.2 nm output spectral 25 dB-bandwidth is measured as 0.3 nm. Edge-mode suppression ratio >60 dB has been achieved and its wavelength stability is 0.05 nm. This fiber laser is able to be used for dense-wavelength-division-multiplex (DWDM) system.