Opening a sizable and tunable band gap in silicene without degrading the carrier mobility is quite desirable for high-speed switching devices. In the present work, the structural and electronic properties of two-dimensional silicene modulated by AsSb with van der Waals (vdW) interaction are investigated by density functional theory with vdW corrections. Notably, there is almost no lattice mismatch introduced in silicene/AsSb heterointerface, which is quite beneficial in comparison with silicene on other substrates. A sizable band gap (213–563 meV) appears in silicene owing to the breaking of the inversion symmetry due to the interface effects, which reveals a potential in applications in such as field effect transistors (FETs) at room temperature. In addition, the nearly linear band dispersion of silicene, guaranteeing the high carrier mobility, can be preserved in silicene/AsSb heterostructures considered in this work. Furthermore, the band gap can be effectively tuned by changing the interlayer distance between silicene and AsSb and, interestingly, an indirect–direct band gap transition occurs. Our results provide a possible direction for experimental fabrication and the applications of silicene-related materials.