The regulation of the valleytronic properties of two-dimensional materials can contribute to the in-depth study of valley physics and improve its potential for applications in valleytronic devices. Herein, we systematically investigate the electronic properties and the modulation of the valleytronic properties in single-layer NbSeCl. Our results reveal that NbSeCl is a semiconductor with a 105.2 meV valley splitting at K and K' valleys in the valence band. Magnetic doping and constructing heterostructures can significantly manipulate the valleytronic properties. Specifically, a large valley polarization of 79.5 meV can be induced by Cr atom doping, and the construction of heterostructures with HfN2 can dramatically increase the valley splitting to 147.5 meV. Intriguingly, the type-II band alignment of the NbSeCl/HfN2 heterostructure may extend the lifetime of valley excitons and realize the valley Hall effect in different layers by doping with electrons or holes.