Electrostatic charging of granular materials in both nature and industry usually takes place in the presence of an external electric field. Thus, it is essential to fully understand the impact charging of a single particle in such situations. Here, we used a variation of Millikan's method to measure the transferred charges by particle-plate impacting under an external electric field. The finite element method was used to determine the electric field at the impact point. We found that impact charges show different polarities under different directions of the vertical component of the electric field: meanwhile, both positive and negative impact charges were present for cases within the shielding cage. A hypothesis based on the theory of electronic energy levels was proposed to explain the total contact potential difference under an external electric field and within the shielding cage. (C) 2019 Elsevier B.V. All rights reserved.