In this paper, the Co_1.2Mn_0.8B compound was prepared by ball milling and solid state reaction. The crystal structure, magnetic properties and magnetocaloric effect of Co_1.2Mn_0.8B compound have been investigated. The results show that the phase structure of the compound is Co₂B single phase with the CuAl₂-type tetragonal crystal structure, space group of I14/mcm. The Co_1.2Mn_0.8B compound undergone second-order magnetic phase transitions from ferromagnetic (FM) state to paramagnetic (PM) state in the vicinity of Curie temperature of 175K, and the thermal hysteresis is about 0.7 K. The maximal magnetic-entropy change (ΔS_M) of the compound reaches to 1.17 J/kg.K in an external magnetic field of 0~5.0 T. and the temperature corresponding to ΔS_Mdoes not change with the variation of the external field. The material has a potential value in the field of magnetic refrigeration and magnetic heat sensor.