Through measurement of saturation magnetic moment and transport of La0.8Sr0.2Mn1-yCoyO3 (y≤02), the mechanism of magnetoresistance (MR) impacted by Co doping is investigated. The results show that Co3+ is in the low-spin state in La0.8Sr0.2 Mn1-yCoyO3(y≤02). The magnetic exchange interactions of the type Mn3+—O—Co3+—O—Mn3+ is weaker than that of the double exchange interaction between Mn3+-Mn4+, and the magnetoresistance near the Curie temperature TC decreases with the increase of Co concentration. On the contrary, low-temperature magnetoresistance increases with the increase of Co concentration owing to the antiferromagnetic exchange coupling between Co2+ and eg itinerant electron.