Two-dimensional InSe possesses good electrical conductivity, intrinsic and structural flexibility, high chemical stability, and a tunable band gap, enabling it to be a promising candidate for flexible and wearable solar cells. Here we construct a lateral p-n junction by partially doping molybdenum trioxide (MoO3) at the surface of the InSe monolayer. Our density functional theory calculations reveal that the strong hybridization between MoO3 and InSe induces a lateral built-in electric field in the partially doped substrate and promotes the effective separation of carriers. Under a large range of external stains, the doped InSe can maintain the direct band gap, and the lateral structure device exhibits power conversion efficiencies over 5% and high carrier mobility around 1000 cm2 V-1 s-1. In particular, a power conversion efficiency of 20.7% can be achieved with 10% compressive strain. The partially doped InSe monolayer is expected to be used as an ultrathin flexible solar cell.