A 1 cm Er/Yb codoped phosphate glass fiber is used to make a short linear cavity fiber laser to get single-frequency,narrow line width and large power output. Relation of the output power to heat sink temperature is measured. When the heat sink temperature is stabilized at 11 ℃,the slope efficiency is 24%;and its output power reaches 79 mW under pump power of 415 mW. The output spectrum shows single longitudinal mode operation at 1534 nm with side mode suppressing ratio (SMSR) above 50 dB. Homodyne measurement gives its line width as narrow as 4.5 kHz. By taking the glass fiber as medium of a low finesse Fabry-Perot (F-P) cavity,refractive index and optic-thermal coefficient of the phosphate glass fiber is measured to be 1.5195 in 1530-1550 nm band and -5.4×10-6℃-1. It is found that the slope efficiency decreases at higher pump power range and the output power increases sub-linearly. This phenomenon and its relation to temperature are discussed qualitatively.