Magnetic field can penetrate a Type-II superconductor to form a vortex lattice (VL) due to the mutual repulsion between the vortices. VL is very important as it contains rich information about the superconductivity (SC). FeSe, despite being the structurally simplest iron-based superconductor, hosts very exotic SC, with the Tc varies from ~ 8 K in the bulk to ~ 110 K in the single-layer limit. The problem is further complicated by the strong interplay between SC, magnetism and nematicity. A recent high magnetic field study indicated a novel field-induced Bardeen–Cooper–Schrieffer (BCS) to Bose–Einstein condensate (BEC) crossover below ~ 1.1 K. Here, we propose to use small-angle neutron scattering to probe the VL in FeSe. By exploring its entire SC phase diagram, we aim to understand the SC and its coupling with nematicity, as well as verify the BCS-BEC crossover scenario in FeSe.