The crystal structure and phase transition in Ni50-xCoxMn38Sn12（x=1, 2, 4, 6, 8） Heusle alloys were investigated by means of structure analysis and magnetism measurements. The results show that the doping of the Co atom affects neither the original structure of Ni-Mn-Sn Heusler alloys, nor the ferromagnetic interaction strengthens in the austenitic state. Furthermore, based on Maxwell equation, the magnetic entropy change （ΔSM） of three samples （x=2, 4, 6） has been calculated. A large magnetic entropy change of 37.09 J/kg K at about 320 K for a magnetic field change of 50 kOe is obtained in Ni48Co2Mn38Sn12 alloy.