A dimension tolerance determining method and a system are disclosed. The method includes: determining initial tolerances of parts in assembled product; generating a plurality of tolerance data sets according to initial tolerances of parts, each tolerance data set containing a calculated part tolerance of each part; inputting the plurality of tolerance data sets into a key parameter generation module to generate a plurality of key parameters corresponding to the assembled product, wherein each key parameter corresponds to one tolerance data set; when the key parameters are located within a design range, calculating a plurality of assembly yield rates based on the tolerance data sets corresponding to the key parameters located within the design range; and selecting the tolerance data set corresponding to one of the assembly yield rates as the tolerances of the parts in the assembled product; wherein the key parameter generation module includes a neural network model.