Rare-earth doped fluorophosphate glass is one of the laser materials that can meet the requirements of wavelength-division-multiplexing (WDM) system and ultra-short pulse system for bandwidth and flat gain. A wide-band, flat gain Yb3+∶Er3+-codoped fluorophosphate glass is investigated, and the calculated full width at half maximum (FWHM) is 51 nm. The study on effective gain cross section shows that there is a flat gain shape between 1530～1580 nm, which proves that the wide-band property of the glass is much better than that of phosphate glasses. The sensitization efficiency of Yb3+ on Er3+ shows that when Yb∶Er ratio is 10∶1, the absorption and emission cross sections of Er3+ reach the maximum of 0.6601 pm2 and 0.7325 pm2 respectively, indicating that in this ratio, energy transfer efficiency reaches the best. Results shows that Yb3+∶Er3+ codoped fluorophosphate glass can be a promising candidate as the media glasses for wide-band, flat gain, high output power lasers and laser amplifiers.