The continuous growth of the world’s population has led to an increased demand for food. Biochar (BC) contains valuable functional groups and nutrients, which have been demonstrated to enhance soil properties and boost crop yield. Nonetheless, unmodified BC exhibits significant alkalinity and a large salt concentration; thus, its efficacy in improving saline-alkali soil remains a matter of contention. Therefore, scholars aim to address the aforementioned limitations by adapting BC. In this paper, a summary is given of the methods used to modify BC and the effects of modified BC on salt-affected soil and saline-alkali soil properties, including pH, nutrients and microbial activity. The effects and principles of different modifiers and modification methods on BC’s physicochemical properties (porosity and functional group content and type, etc.) were revealed. Furthermore, the principle of soil improvement by modified BC varies depending on the modification method. Although modified BC can effectively enhance salt-affected soil, its excessive application and improper selection of modifiers can aggravate Na+ toxicity or introduce other pollutants into salt-affected soil. Therefore, using modified BC with care is imperative according to the actual situation. In this review, we provide a detailed introduction to the methods and principles of BC modification for saline-alkali soil. Furthermore, we identify the shortcomings and future research directions in this field. These insights are valuable when choosing proper BC modification methods for salt-affected soil.