According to the problem of slow convergence using existing PPP models, a modified PPP model was proposed. With this new model, a code-phase ionospheric-free combination is used to lower the code observation noise level, and a code-phase geometry-free combination is used to mitigate orbit errors. The new PPP model composed by the new combinations not only has a lower measurement noise, but also can reduce the impact of orbit error. Results show that the performance of PPP solution is improved by introducing the new combinations. For the 0.5 h duration observation data sets, the convergence percentage by using MUC model is increased by 37.6%, 4.2% and 235.9% compared with the Un-Combined (UC) model, UofC model and Un-Difference ionosphere-free combined (UD) model, respectively. Moreover, the positioning accuracies of the modified PPP model agree with those of the UofC model, which are better than those of the UC and UD models. The accuracy of zenith tropospheric delay agrees with those of the UC model, and these are better than those of the UofC and UD models.