In zero-gap semimetallic silicene, introducing a sizable band gap without degrading its high carrier mobility is vital to its application in optoelectronic devices. Herein, we design a novel atomically thin system based on silicene and arsenene nanocomposites (Si/As heterostructure), which could open a direct band gap of about 125 meV at the K point in silicene. Moreover, its band gap is linearly controllable over a wide range even with a semiconductor–metal transition by the external electric field (E_⊥), with an impressive band gap of up to 328 meV at E_⊥ = −0.9 V Å⁻¹. Additionally, the Si/As heterostructure can exhibit pronounced optical absorption in the far infrared range. The binding energy of the first bright exciton is as large as 623 meV, which can be significantly increased with an enhanced E_⊥. The tunable bandgap together with a superior optical absorption makes the Si/As heterostructure a potential candidate for nanoelectronic and optoelectronic applications.