The invention relates to a determination method for the maximum allowable input quantity of a nitrorgenous fertilizer in a watershed scale. The method comprises the following steps: (1) selecting a typical research region; (2) constructing a model of non-point source pollution of the typical research region and carrying out simulation verification; (3) establishing a response relationship between the input quantity of the nitrorgenous fertilizer and the output concentration of the nitrorgenous fertilizer; (4) affirmatively ensuring a safe total nitrogen concentration limiting value of quality of surface water in a target research region according to environmental planning and environmental partitioning water quality demands of the target research region; and (5) determining the maximum allowable input quantity of the nitrorgenous fertilizer of the target research region. The method provided by the invention has a very good application prospect in the technical field of prevention and control management of agricultural non-point source pollution in the watershed scale.