Abstract: In order to study CO2 emission law of field soil for regulating carbon balance, seasonal variation of CO2 emission of maize and soybean field was investigated by continuous observation. Taking dry farmland soil as the research object, the carbon balance of farmland ecosystem was estimated. The results showed that: 1) Farmland soil CO2 emission flux presented a single peak curve with seasonal variation. CO2 emission flux reached the maximum in July. Straw mulching significantly increased the farmland soil CO2 emission; Maize or soybean plantation had little effect on soil CO2 emission. 2) Seasonal variation of soil temperature was consistent with that of soil CO2 emission, the relationship between soil CO2 emission flux and soil temperature could be well simulated by exponential equation and quadratic equation. However, exponential equation was better than quadratic equation, the correlation of soil temperature of 20 cm was the highest, which of 5 cm was the lowest. 3) Maize and soybean fields were usually the "sink" of atmospheric CO2, the average carbon-sink of maize-maize- soybean rotation (three years) was 4.53 t/hm2.