There is a substantial demand for soil amelioration in coastal saline-alkali land. Carbonizing straw is an eco-friendly and cost-effective method for producing soil amendment products. However, the effect of direct application of biochar on saline-alkali land improvement is not good or even harmful. Therefore, a new kind of humic acid magnetic biochar (HAFO) was prepared by pyrolysis, oxidation and activation, and its properties and pot experiment results were evaluated. The results showed that pyrolysis and oxidation effectively load nano-Fe 3O 4 onto biochar while reducing its particle size to the sub-micron level. In this process, the properties of biochar are improved (surface oxygen-containing functional groups, polarity, nutrient elements, etc.), making it more conducive to saline-alkali land improvement. The pot experiment showed that rice growth and development were affected by soil organic matter content, cation exchange capacity, salinity and nutrient content (p<0.05). Biochar alleviated the effects of salt stress on rice by improving soil properties, that thus promoted the growth and development of rice. The effect of biochar on the improvement of saline-alkali soil and on the growth of rice is mainly affected by its own properties, and appropriate modification methods can make better utilization of straw resources. The HAFO prepared in this study is an economical and efficient soil conditioner for saline-alkali land, which is of great significance for world food security.