We investigate, based on laser performance, the fractional thermal loadings in a ${\mathrm{Yb}}^{3+}/{\mathrm{Er}}^{3+}$ codoped phosphate glass laser. The results indicate that fractional thermal loading increases with ${\mathrm{Er}}^{3+}$ doping concentration nonlinearly. Above an ${\mathrm{Yb}}^{3+}$ doping level, the increase of ${\mathrm{Yb}}^{3+}$ cannot promote laser performance but induces a more serious heat effect. We investigate the main factors that simultaneously affect laser and thermal performances. The results show that the ratio of laser output power to heat is sensitive to beam overlap efficiency, the laser extraction efficiency, especially to quantum efficiency of laser material. The optimized doping concentrations of ${\mathrm{Yb}}^{3+}$ and ${\mathrm{Er}}^{3+}$ are deduced.