Van der Waals(vdW)layered two-dimensional(2D)materials,which may have high carrier mobility,valley polarization,excellent mechanical properties and air stability,have been widely investigated before.We ex-plore the possibility of producing a spin-polarized two-dimensional electron gas(2DEG)in the heterojunction composed of insulators MoSi2N4 and VSi2N4 by using first-principles calculations.Due to the charge trans-fer effect,the 2DEG at the interface of the MoSi2N4/VSi2N4 heterojunction is found.Further,for different kinds of stacking of heterojunctions,lattice strain and electric fields can effectively tune the electronic struc-tures and lead to metal-to-semiconductor transition.Under compressive strain or electric field parallel to c axis,the 2DEG disappears and band gap opening occurs.On the contrary,interlayer electron transfer enforces the system to become metallic under the condition of tensile strain or electric field anti-parallel to c axis.These changes are mainly attributed to electronic redistribution and orbitals'reconstruction.In addition,we reveal that MoSi2N4/VSi2N4 lateral heterojunctions of armchair and zigzag edges exhibit different electronic proper-ties,such as a large band gap semiconductor and a metallic state.Our findings provide insights into electronic band engineering of MoSi2N4/VSi2N4 heterojunctions and pave the way for future spintronics applications.