The effect of Co addition on the density of states (DOS) distribution of Ni-Mn-Ga ferromagnetic shape memory alloys was investigated by first-principles plane-wave pseudo-potential method based on the density functional theory. The mechanism of effect of Co element on martensitic transformation of Ni-Mn-Ga alloy was clarified. Results show that with Co content increasing, more Co3d-Mn3d hybridization replaced Ni3d-Mn3d, improving parent phase stability and decreasing martensitic transformation temperature. In addition, it is noted that Co addition has little effect on majority-spin DOS of Ni-Mn-Ga-Co parent phase, but changes minority-spin DOS significantly.