Abstract: The influence of the farmland micro-terrain on the actual operation pressure of the emitter was analyzed firstly. The more uneven the farmland was, the more deviation between the actual operation and designs pressure of the emitter was. When the designs pressure of the emitter was below to sectional difference of farmland elevation, the flow of the emitter would be equal of zero. The calculation formula of the flow deviation rate of the emitter, which was from the farmland micro-terrain, was proposed and developed by the formula of Tailor. By method of described statistic and T-test, the surplus of the formula of Tailor was analyzed. The result indicated that the surplus could be neglected and the calculation formula of the flow deviation rate of the emitter, which was from the farmland micro-terrain, was determined by simple form.