We study the electrical and optical properties of the van der Waals (vdW) heterojunction Be2C/graphene by first principles. The constructed heterojunction exhibits p-type Ohmic contact. It is found that the varied interlayer distance and biaxial stress can influence the electronic structure and charge transfer of heterojunction. While the small variety of the difference of electrostatic potential of the heterojunction induces the small change of charge transfer at the contact interface, so the p-type Ohmic contact can still be formed. Meanwhile, the decreased interlayer distance and tensile strain can enhance the optical absorption of heterojunction. Then we calculate the external quantum efficiency of the optoelectronic device based on Be2C/graphene that reaches to 27.8%. This work provides a way to design electronic and optoelectronic devices based on Be2C/graphene in the future.