Abstract: Based on the estimation of polynomial coefficients, the inter-frequency bias of GPS satellites and receivers' PCB between P-codes of GPS pseudoranges are combined,and the ground-based GPS observations areased to construct regional VTEC model.Data analysis indicates that,while short-term(i.e.4 hours') GPS observations are used,the result of inter-frequency bias is not stable,and polynomial VTEC model has systematical error and shows large errors in the edge of the covered region.However,if the piecewise constant modeling is used to estimate several sets(i.e.6 groups) of VTEC polynomial coefficients and one group of the combined inter-frequency bias while using 24-hour's GPS dual frequencies' observations,the result of differential P-code bias is consistent and stable,and the new polynomial VTEC model is more accurate.