In a superhigh magnetic field, direct Urca reactions can proceed for an arbitrary proton concentration. The magnetic field is the main energy source of all the persistent and bursting emissions observed in anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs). Employing the assumption of micro-state number, we deduce the formula of electron Fermi energy EF (e) in superhigh magnetic fields. Based on observing and introducing relevant parameters introduced, a approximate way of calculating the X-ray luminosity of a Magnetar LX in any superhighmagnetic field is introduced. Copyright owned by the author(s).