Abstract: With the increasing diversification of raw materials used for organic fertilizer production, in addition to traditional feedstocks such as livestock and poultry manure, various processed organic wastes, including distillers’ grains, glutamate residues, and rapeseed cake, have also become important raw materials for commercial organic fertilizer production. These evaluation-category feedstocks generally contain relatively high levels of nutrients, soluble salts, low-molecular-weight organic acids, ammonium compounds, and other potentially phytotoxic substances. When the germination index (GI), which is used in the Chinese Organic Fertilizer standard (NY 525-2021) to characterize compost maturity, is applied to assess the maturity of such feedstocks, relatively low GI values are often obtained under conventional extraction conditions. Therefore, applying the same extraction ratio used for conventional composting materials may not be appropriate, and a targeted GI-based evaluation method for these evaluation-category feedstocks is still lacking. Accordingly, this study aimed to optimize the GI-based maturity evaluation method for organic fertilizers produced from evaluation-category feedstocks and to determine suitable extraction ratios and seed incubation times for different types of raw materials. Eight representative categories of evaluation-category feedstocks used in organic fertilizer production, including distillers’ grains, glutamate residues, rapeseed cake, and other typical materials, were selected as the research objects. Five compost-to-water extraction ratios, namely 1:30, 1:50, 1:100, 1:200, and 1:300, were established, and two seed germination incubation periods, 48 h and 72 h, were compared. Under these different treatment conditions, the germination index (GI), electrical conductivity (EC), and pH of the compost products derived from different feedstocks were determined to evaluate the effects of extraction ratio and incubation time on maturity assessment. The results showed that different categories of evaluation feedstocks required different extraction conditions for GI determination. For high-salinity raw materials, such as glutamate residues and yeast metabolites, a relatively high extraction ratio of 1:200 (compost:water) combined with a 72-h radish seed incubation period was more suitable for maturity evaluation. Under lower extraction ratios, the relatively high salt concentrations in the extracts could strongly inhibit seed germination and result in low GI values, thereby affecting the assessment of compost maturity. For materials such as rapeseed cake and mushroom residues, a moderate extraction ratio of 1:100 combined with a 72-h seed incubation period was more appropriate. In addition, when evaluation-category feedstocks accounted for less than 15% of the total raw materials used for composting, extraction ratios of 1:30–1:50 could be adopted, indicating that the influence of these feedstocks on GI determination was reduced when they were used at relatively low proportions in mixed composting systems. Overall, the results demonstrated that the extraction ratio and seed incubation time should be adjusted according to the characteristics of different evaluation-category feedstocks rather than using a single fixed condition for all raw materials. By systematically comparing gradient extraction ratios and different seed incubation periods, this study identified suitable GI determination conditions for several representative evaluation-category feedstocks. These findings provide technical support and a scientific basis for improving the maturity evaluation of organic fertilizer products produced from evaluation-category organic wastes in China, and may also contribute to the further refinement and practical application of GI-based maturity assessment methods for diversified organic fertilizer feedstocks.