Er3+/Yb3+ codoped phosphate glass is prepared using a high-temperature melting method. The cladding glass with different compositions has been designed and prepared, and the rod-in-tube technique is employed in fabricating Er3+/Yb3+ codoped core-cladding heterogeneous glass fiber. Under the excitation of 980 nm wavelength laser, when the pumping power increases to 457.1 mW, the relative and internal gains of the optical fiber are 32.3 and 15.0 dB at 1535.7 nm, and a maximum internal gain-coefficient of 2.6 dB/cm is achieved.