Ni–Mn–Sn-based magnetic refrigeration materials have a coupling ability of structure transformation and magnetic transition to improve their magnetocaloric effects. In this work, the influence of the Co doping on Curie temperature, structure and magnetocaloric properties of Ni 43- x Co x Mn 46 Sn 11 ( x = 0, 1, 2, 3) alloys prepared by melt-spinning were investigated by means of X-ray diffraction, Differential Scanning Calorimetry and Vibrating Sample Magnetometry. It was found that the crystal structure of samples changes from 4O modulated martensite to L2 1 cubic austenite with increasing temperature. The magnetic phase diagram of Ni 43- x Co x Mn 46 Sn 11 system was constructed from thermomagnetic curves. Increasing Co content enhanced the Curie temperature of L2 1 cubic austenite from 279 K to 354 K, however, shifted the transition temperature of martensitic down to lower temperature, 170 K. As for the sample with x = 3, it only has ferromagnetic–paramagnetic second order magnetic transition, and no structural transformation was observed. The sensitivity of martensitic transformation induced by magnetic field can be improved up to − 3.52 K · T −1 . Under 0–5 T, the maximum values of magnetic entropy change, refrigerant capacity and the adiabatic temperature change are improved by 60.5%, 59.0% and 17.5%, respectively. The results indicate that the relatively large martensitic transformation sensitivity and magnetocaloric effects make these alloys good candidates in the range of 150–300 K.