Whistler‐mode waves are electromagnetic waves pervasively observed in the Earth's and other planetary magnetospheres. They are considered to be mainly responsible for producing the hazardous radiation and diffuse aurora, which heavily relies on their properties. Density irregularities, frequently observed in the Earth's magnetospheres, are found to change largely the properties of whistler‐mode waves. Here we report, using Van Allen Probes measurements, whistler‐mode waves strongly modulated by two different density enhancements. With particle‐in‐cell simulations, we propose wave trapping caused by field‐aligned density irregularities (ducts) may account for this phenomenon. Simulation results show that whistler‐mode waves can be trapped inside the enhanced density ducts. These trapped waves remain quasi‐parallel and usually get much larger amplitudes than those unducted whistler waves during propagation away from the magnetic equator, and tend to focus at a spatially narrow channel, consistent with observations. Our results imply density irregularities may be significant to modulate radiation‐belt electrons. Key Points Particle‐in‐cell (PIC) simulations demonstrate whistler‐mode waves can be trapped inside the enhanced density ducts which can interpret satellite observations These trapped waves remain quasi‐parallel and get much larger amplitudes during propagation toward high latitudes These trapped waves focus at a spatially narrow channel inside the enhanced density ducts