Compared with the experiment results,the atomic moment of Co50Fe25-xMnxSi25 dependending on composition and half-metalic of these alloys have been investigated by GGA + U method. It has been found that with the increase of Mn content, Co50Fe25-xMnxSi25 alloys always maintain a high ordered L21 structure and the total moment of these alloys decrease linearly which is mainly attribute to the reduction of Co atomic moment. It has also been found that with the increase of Mn content,Co50Fe25-xMnxSi25 alloys maintain 100% spin polarization and Fermi level move from the edge of the energy gap to the middle of the energy gap while the gap-width has changed.