The influence of cobalt dopant on the photoluminescence and magnetic properties of SnO2 nano-powder is studied in detail. The cobalt dopant has little influence on the photoluminescence peaks' positions. However，the intensity ratio of ultra-violet to blue emission band is reduced with the increase of cobalt doping. The ferromagnetism is absolutely absent when the cobalt dopant level reaches 0.02. The magnetic interaction mechanism is discussed. Due to the non-uniform distribution of doping，and the coupling between the polarized spin on defects and the dopants，the anti-ferromagnetic interaction destroys the ferromagnetism.