The effects of different holding times on the biochar yield,composition,pH,electrical conductivity and pore structure of five kinds of straws including rice,wheat,corn,rape and cotton were investigated.Correlation between the physical and chemical properties of different biochars was analyzed.The results showed that effects of straw type and holding time on the physicochemical properties of biochar fertilizer were significant (P<0.05).The biochar yields and the carbon conversion efficiency of straws ranged from 41% to 61%,and 53% to 65%,respectively.With the increase of holding time,the biochar yields decreased,while the pyrolysis degree and the fixed carbon contents of biochars increased.The pH and electrical conductivity increased with the increase of holding time.The holding time was negatively correlated with carbon yield,H,O contents and volatile matter content of biochars,and positively correlated with fixed carbon content.The optimal holding time is 60-90 min,and the biochars obtained under this condition has a rich pore structure with smooth surface.The biochars with lower H/C and higher pH and electrical conductivity can be used as a good carbon-based fertilizer additive material.