The electron confinement in an electron cyclotron resonance ion source is considered a critical factor that has a great influence on the working efficiency. In this paper, a three-dimensional electromagnetic particle-in-cell code is presented for simulations of the electron confinement in minimum-B structures. Electron dynamics under the effect of electromagnetic fields is modeled through a leapfrog scheme by solving Maxwell's equations and the particle equations of motion iteratively. Some characteristics of electron magnetic confinement are obtained through a benchmarking example. Preliminary results of the simulation show good agreement with the well-known effects.