The crystal structure, electronic configuration and electronic structure of siderite FeCO3 are studied by first-principles calculations through the plane wave pseudo-potential method. The real antiferromagnetic (AFM) spin ordering state is considered. The pressure increases up to 500 GPa under hydrostatic pressure condition. FeCO3 transforms from high spin (HS) AFM state to low spin (LS) nonmagnetic (NM) state in a pressure range between 40 and 50 GPa, accompaned with a volume collapse of 10.5%. Siderite FeCO3 is insulating before and after the phase transition, but the 3d electrons of Fe2+ ions for the LS-NM state are more localized, The band gap increases with pressure increasing, and the LS-NM state moves into a more strong ionic state and no metal-insulator transition (MIT) occurs.