Magnetic properties of transition metal Co doped Fe₃Si alloy are studied with first-principles pseudo-potential plane wave method based on density functional theory (DFT). It shows that magnetism of Fe_3-xCo_xSi mainly results from transition metal elements Fe and Co. Strong-magnetism of Fe_B atom is revealed compared with weak-magnetism of A- and C-site atom. Total magnetic of Fe_3-xCo_xSi decreases slowly in 0≤x≤0.75, but increases rapidly in 0.75≤x≤1.5. Fe_A,C moment shows similar trend. Magnetic moment of Co atoms increases slowly as Co content increases. Change of atomic magnetic moment is related to charge transfer of spin up and down direction.