Abstract: Taking two kinds of typical coastal saline-alkali soil（silt loam and sandy loam）as the research object,different particle sizes（1～2 mm,＜1 mm）and different addition amounts of biochar（0,2.5%,5%）are applied. Then the pot experiment of tomato is carried out to determine the physical and chemical characteristics of soil and the yield and quality of tomatoes so as to understand the function and effect of biochar on improving coastal saline-alkali soil.The results show that:（1） In addition to the 2.5% addition treatment in sandy loam,biochar significantly reduces soil bulk density,improves the three-phase ratio of soil,reduces conductivity and sodium adsorption ratio,increases soil nutrient content. And the soil physical and chemical properties of 5% biochar treatment are better than that of 2.5% biochar treatment.The effect of 1～2 mm biochar on reducing bulk density and conductivity is better,and the effect of ＜ 1mm biochar on increasing part of nutrient content is better.（2） Tomato yield is the highest in silty loam with 5% application of 1～2 mm particle size biochar,which is 20.6% higher than the control. Tomato yield is the highest in the sandy loam with 5% application of ＜1mm particle size biochar,which is 19.8% higher than the control. The mass concentrations of soluble solids,soluble sugar,vitamin C and titratable acid in tomato increases with the application of biochar,but there is no significant correlation with the particle size of biochar.