We numerically investigate the first-order electroweak phase transition in the background of a hypermagnetic field with three-dimensional lattice simulation. The generation of baryon asymmetry is observed, and we present the relationship between baryon number asymmetry and magnetic field strength and its helicity. We find the magnetic field strength required to achieve the correct matter-antimatter asymmetry is about ${10}^{-17}\sim {10}^{-14}$ Gauss at present, depending on the correlation length of the helical magnetic field. This study provides a mechanism for explaining the baryon number asymmetry with cosmic magnetic fields.