Isotope data of key pollutants are needed for source apportionment analysis in natural ecosystems. Isotope data for cadmium (Cd), a rare and dispersed but toxic element, are very limited mainly due to its low contents in the natural environment. In this study, for samples with low Cd content but high organic matter content, a dry ashing pre-treatment method was proposed to effectively enrich Cd to meet the requirements of its isotope analysis (Cd content > 20 ng). This method was applied to soils, sediments, biological tissues, and coal, with a maximum digestion weight of 1.67 g or more. With the assistance of quartz microfiber filter membranes during dry ashing, good Cd recovery (91.6–108.0%) and significant removal of organic matter were achieved. Mass bias was corrected by combining the silver external standard (Ag-doping) method with the sample-standard-bracketing (SSB) method, and Cd isotope measurements were performed under wet plasma conditions (with an accuracy of 0.060‰). Cd isotopic compositions obtained by the dry ashing method were compared with those obtained by the high-pressure bomb digestion method, and the differences ranged from −0.056 to 0.076‰, indicating that the dry ashing with wrapped membranes method did not cause any fractionation of Cd during the sample pre-treatment process. Meanwhile, the tolerance of tin (Sn) and zinc (Zn) elements provided a significant advantage under wet plasma conditions over dry plasma conditions without reducing the tolerance of molybdenum (Mo) and zirconium (Zr). The method developed in this study should enhance the application of Cd isotopes in investigating Cd cycling in primary ecosystems and organisms.