A novel evaluation theory for sustainable agricultural development was developed using the standardized Euclidean distance method. It contains an agriculture sustainable development model, an evaluation function, and agriculture sustainable development ranks. It includes a goal system, a zero system, and an index frame. Wuzhong County in Ningxia Hui Autonomous Region (WZC) was used as a case study area between 2000 and 2012. The results showed that the agricultural sustainable development exponent (ASDE) in WZC slowly increased over the period from 0.65 to 0.70. The positive features of the method are: (1) the methodology was reliable; (2) the agriculture sustainable development model was constructed using the goal system and the zero system, which provides a reference system for sustainable agricultural development and improves evaluation controllability; (3) the index weights were embedded in the evaluation function, which removes the errors or mistakes made by the expert scoring method and avoids having to reassign index weights because of indicator addition or subtraction; and (4) the agriculture sustainable development exponent was related to the centesimal system by a function model transformation, which gives a very clear figure conception because the primitive value from the Euclidean distance is difficult to understand. It is important to compare systems and method application, and this model suggests that the low ASDE in WZC is mainly due to high nitrogen fertilizer application rates, low straw application rates, the compound or special fertilizer ratios, the proportion of the land taken up by conservation farming areas, soil organic matter levels, and income per rural person.