The magnetic field formula of a point charge in relativistic form is derived by using the Lorentz transformation. On this basis，we estimate the background magnetic field in relativistic heavy - ion collisions by aking consideration in the longitudinal thickness of nucleus. The numerical calculations of Au -Au collisions in RHIC energy regions and Pb-Pb collisions in LHC energy regions are carried out.Then，we discuss mainly the energy dependence of the magnetic field at original point and give an intuitive，qualitative theory explanation.