In the framework of effective mass envelope function approximation, the impurity states are calculated in InGaAsP/InP core–shell quantum dots by plane wave expansion method, and the effect of electric field is considered. The impurity binding energies in 1s and 2p  ±   states, and the impurity transition energy between 1s state and 2p  ±   state are calculated and analyzed in detail with the change of shell thickness, core radius, impurity position, and electric field strength. The results show that the impurity states in the core are stable when the shell thickness is more than 0.4a^*. The binding energies and transition energies decrease with the increase of core radius. The applied electric field destroys the symmetrical distribution of binding energy and transition energy about the core center. The increase or decrease of the binding energy and transition energy depend upon the position of impurity and the direction of electric field.