Donor impurity states in a quasi-conical quantum dot are theoretically investigated. The confinement potential is considered within the framework of the model with infinite walls. The problem is considered because the impurity is located at the vertex of a quasi-conical quantum dot. Based on the hydrogen atom model, the impurity and the electron's interaction potential is Coulomb potential. Because of the complexity, the problem is solved using the approximate finite element method (FEM). The FEM method is used to compute the energy states and wave functions of the system both in the presence and absence of the donor impurity. Based on the obtained results, the dependences of the impurity binding energy on the geometric parameters of the quasi-conical quantum dot and the probability density of the electron cloud distribution in the quantum dot are studied.