Belonging to the technical field of agricultural environmental protection, the invention relates to a calculation method for a farmland nitrogen input threshold. The calculation method for the farmland nitrogen input threshold is carried out according to the steps of: conducting comparison planting and data statistics; utilizing the statistical result to determine the farmland crop's output nitrogen threshold and the farmland's environmental nitrogen input threshold; determining the farmland soil type according to the crop's output nitrogen threshold and the regional farmland's environmental input nitrogen threshold, dividing the soil fertility level, and correcting the determined nitrogen input threshold; and finally, calculating the farmland nitrogen input threshold. According to the invention, the farmland nitrogen input threshold is calculated based on the crop's output nitrogen threshold and the regional farmland's environmental input nitrogen threshold, a nitrogen fertilizer can be reasonably used to reach the production increase effect, at the same time, wasting is avoided, the influence to the soil ecological environment can be avoided, balance between supply and demand can be realized, rational use of soil is promoted, and the contradiction between fertility and pollution and the contradiction between yield increase and energy saving in long-term use of farmland are solved. Thus, the method provided by the invention plays an important role in leading the agricultural industry to sustainable and healthy development.