Abstract: As a key carrier in the mineral element cycle within the “soil-plant-food chain”, the mineral element content in plants is of great significance for the evaluation of agricultural product nutritional value and the assurance of food safety. However, plant samples are characterized by high organic matter content and complex components, which lead to matrix interference. For sample pretreatment, wet digestion is prone to generating toxic gases, while dry digestion tends to cause some compounds to escape as volatile substances—both issues affect the accuracy of detection results. Based on the optimization of wet digestion, this study established a method for determining 8 mineral elements (Al, Ca, Cu, Fe, K, Mg, Mn, and Zn) in plant samples using microwave digestion with dilute nitric acid-hydrogen peroxide combined with inductively coupled plasma-optical emission spectrometry (ICP-OES). Meanwhile, a relative carbon concentration (RCC) of less than 13% was introduced as an indicator to evaluate digestion efficiency. The “single-factor screening-response surface methodology” was adopted to investigate sample treatment conditions such as nitric acid concentration, digestion time, and hydrogen peroxide dosage. Under the optimal conditions (nitric acid concentration of 3.5 mol/L with a volume of 5 mL, digestion time of 15 min, and hydrogen peroxide dosage of 2.5 mL), the RCC was consistently less than 13%, and organic substances such as cellulose and lignin in the samples were completely decomposed. The determined values of Al, Ca, Cu, Fe, K, Mg, Mn, and Zn in the certified reference materials were basically consistent with the standard values. The limits of detection (LOD) for these elements were 0.75, 1.33, 0.12, 0.14, 0.67, 2.12, 0.14, and 0.22 μg/g, respectively. The accuracy (relative error, RE) was less than 12%, and the relative standard deviation (RSD) was less than 10%, which meets the requirements specified in Technical Requirements for Sample Analysis in Eco-Geochemical Evaluation (Trial) (DD2005-03). There was no significant difference between the detection results of this method and those of ICP-OES and ICP-MS methods reported in the literature. Verified by actual samples, this method enables the quantifiable evaluation of digestion efficiency through the RCC index, reduces reagent consumption and environmental pollution, and provides a referable pretreatment approach for element determination.