In order to understand the mechanism of biochar affecting soil fertility, six biochar levels, 0, 10, 30, 50,70 and 90 t/ha were set up in pot experiment, and four potassium indexes of Lou soil were tested by flame photometer: water soluble potassium, exchangeable potassium, available potassium and total potassium. The results showed that: after biochar was added to Lou soil, the contents of water-soluble potassium, available potassium, exchangeable potassium and biochar showed an y=ae^bx index relationship, and soil pH was significantly correlated with the contents of available potassium and exchangeable potassium, but not with the contents of water-soluble potassium and total potassium. Adding biochar to Guanzhong Lou soil can significantly increase the content of soil available potassium, water-soluble potassium and exchangeable potassium. Taking into account the potassium ion, pH and economic factors, 50 t/ha biochar was applied 30 days in advance in Lou soil is the best.