【Objective】A reasonable supply of magnesium is an important guarantee for high-quality tobacco production. The current soil magnesium fertilizer application in tobacco growing areas is mainly determined according to the level of soil available magnesium content and its spatial distribution. However，the practical impact of the ratio of soil available calcium content to available magnesium content（Ca/Mg ratio）on magnesium supply is rarely considered. 【Method】Taking Bijie tobacco growing area as an example，the level of available magnesium content，Ca/Mg ratio and spatial distribution contours were obtained based on a large number of soil samples. Then，the suitable grade zoning of magnesium fertilizer was delimited through graphic superposition and grid operation. 【Result】The results showed that the average content of soil available magnesium in Bijie tobacco growing area was 278.1 mg· kg^-1. The content shows great fluctuation in space，which was caused by the parent rock，soil type，elevation and topographic location. Also，the average Ca/Mg ratio in this area was 14.5，and was at a high level. Moreover，the spatial fluctuation of the ratio was also large，and mainly related to the spatial distribution of soil-forming parent rock and soil types. From spatial interpolation contours of available magnesium content，the areas with high content were mainly concentrated in the east and southwest，while areas with low content were mainly distributed in the middle north and northwest. The available magnesium content was dominated by the suitable grade，which accounted for 69% of the whole area. Moreover，the area of lacking and rich grades accounted for 19.0% and 10.5% respectively，and the areas of extremely lacking and extremely rich grades were both small. From the distribution of soil Ca/Mg ratio，the study area was dominated by two grades of 10～20 and 5～10，while areas covering > 20 and < 5 were relatively small. On the whole，the Ca/Mg grades were characterized by interphase distribution in space. In the application grade map of soil magnesium fertilizer obtained from available magnesium content and Ca/Mg ratio，the most suitable application areas were mainly concentrated in the north-central，southeast and northwest regions. Also，the unsuitable grades were mainly distributed in some parts of the southwest and northeast，while the areas of extremely unsuitable grade were very small，and mainly scattered in the southwest. Among all the grades，the areas of suitable and more suitable grades of magnesium fertilizer application accounted for 69.0% and 10.5% respectively. The unsuitable application area accounts for 19.0%，while the proportion of extremely suitable and extremely unsuitable areas account for only 1.3% and 0.2% respectively. 【Conclusion】The results indicate that the Ca/Mg ratio of soil is an important factor to be considered when formulating the application scheme of magnesium fertilizer. If the influence of the Ca/Mg ratio on soil magnesium supply is not considered，the appropriate grade zoning of magnesium fertilizer will have a large deviation，which will have an adverse effect on soil fertilization in tobacco growing areas. It can be seen that comprehensive consideration of soil available magnesium content and Ca/Mg ratio is an important prerequisite for scientifically formulating regional magnesium fertilizer application scheme.