The salinized gray desert soil （S） in Xinjiang and the acid red soil （A） in Hubei were used to study the response of nutrient availability and fertility characteristics to biochar （BC） in the two typical soils of acid and alkali in China. Four levels of biochar （maize straw as raw material） including C0 （0%）， C0.5 （0.5%）， C1 （1%） and C2 （2%） were set. A soil cultivation experiment was conducted for 60 days. 4 dynamic sampling periods including 1 d， 10 d， 30 d， 60 d were selected. The basic physical and chemical properties， salinity， the content of nutrient， and the activity of extracellular enzymes in soil were measured. The response of nutrient availability and fertility characteristics of two typical soils of acid and alkali to different levels of biochar application was analyzed. The results showed that the application of biochar improved the physical and chemical properties of the two soils， and increased the content of nutrient in the soil. C2 significantly increased the pH of red soil by 0.14 units and decreased the pH of saline soil by 0.18 units. Compared with C0， C2 significantly increased the content of organic matter and available potassium in acid red soil and saline soil by 71.25%，59.65% and 142.31%，36.85%， respectively. Compared with C0， the maximum increase in the content of water-soluble potassium in red soil was 238.10% after 10 days of cultivation under C2， and the maximum increase in the content of water-soluble potassium in saline soil was 47.50% after 1 day of cultivation. The maximum increase in the content of exchangeable potassium in red soil was 127.88% after 60 days of cultivation， while the maximum increase in the content of exchangeable potassium in saline soil was 31.58% after 1 day of cultivation. C2 reduced the content of non-exchangeable potassium in saline soil by 7.62% after 60 days of cultivation. The application of biochar significantly increased the content of water-soluble magnesium and the ratio of potassium to sodium（K⁺/Na⁺） in red soil. The application of biochar significantly reduced sodium adsorption ratio （SAR） and the content of water-soluble sodium in saline soil. C0.5 significantly increased the content of water-soluble calcium and water-soluble magnesium by 4.90% and 2.80% compared with C0， respectively. The activities of extracellular enzymes in the two typical soils decreased with the application of biochar. It was found through redundancy analysis that extracellular enzymes in soil were positively and strongly correlated with the content of available potassium， water-soluble potassium， and SAR. In conclusion， the application of improved the availability of nutrient in two typical soils， and the content of available potassium and water-soluble potassium were the key driving factors of soil fertility. The strong adsorption of biochar and the replacement between salt ions reduces the saline alkali stress effect of sodium ions， has a positive effect on improving the fertility characteristics of two typical soils.