Yb-Er codoped Na2O-Al2O3-P2O5-xSiO2 glasses containing 0-20 mol% SiO2 were prepared successfully. The addition of SiO2 to the phosphate glass not only lengthens the bond between P^{5 } and non-bridging oxygen but also reduces the number of P=O bond. In contrast with silicate glass in which there is only four-fold coordinated Si^{4 }, most probably there coexist [SiO4 tetrahedron and [SiO6] octahedron in our glasses. Within the range of 0-20 mol% SiO2 addition, the stimulated emission cross-section of Er^{3 } ion only decreases no more than 10%. The Judd-Ofelt intensity parameters of Er^{3 }, \Omega_{2} does not change greatly, but \Omega_{4} and \Omega_{6} decrease obviously with increasing SiO2 addition, because the bond between Er^{3 } and O^{2-} is more strongly covalently bonded.