According to China Irrigation Industry Technological Specifications, in the design of drip-irrigation system the total flow deviation rate was calculated considering only one factor of hydraulic deviation rate, resulting in failing to meet the actual conditions. With the production development and technology progress, in order to improve the design accuracy, it is necessary to take into consideration the effects of manufacture flow deviation rate and elevation flow deviation rate on the total flow deviation rate. Since there existed some questions and incredible results in the previous literature, the total flow deviation rate of drip-irrigation system was studied considering the three deviation rate factors. On the basis of the elevation flow deviation rate, the manufacture flow deviation rate of driper was defined, the formula for calculating total flow deviation rate was derived under the condition of the most unfavorable combination of the three factors, i.e. hydraulic flow deviation rate, manufacture flow deviation rate and elevation flow deviation rate. Finally, the permitted values of the total flow deviation rate were recommended.