We propose a theoretical model for magnetar giant flare to explain the flaring activity on 2004 December 27 from SGR1806-20 comprehensively. A global rearrangement is expected by the magnetic reconnection that requires explaining the giant SGR flares. In this paper we propose two regions of flares: preflare on the surface of magnetar and main burst at a distance of light cylinder radius. Acquiring the maximum potential drop on the magnetar surface, adopting space charge limited flow model, and using magnetic field B ≈ 10¹⁵ G, the luminosities of flare energies release for the preflare phase and main burst phase are found to be in the order of 10⁴¹ erg·sˉ¹ and 10⁴⁴ erg·sˉ¹ respectively, conforming to magnetar burst energy and flare temperature is determined by considering black body radiation.