A model MDS-2002A microwave digestion sample preparation system with auto control pressure in obturation was developed for the determination of microelements in clearing-up plant's ash and environment soil sample by ICP-AES. The authors discussed sample preparation result influenced by mixed acid system, scale of mixed acid dispensation, proportion between solid and liquid and time of microwave clearing. Excellent project was A2B1C2, work procedure 2 in microwave process. When the dispensation scale was 6 : 2 : 1 : 1 and mixed acid system was HNO3-HCl-HF-HClO4, the result was the best in 10 min of microwave clearing at highest power. The reagent was dried at constant temperature electric heating board, with its salinity being dissolved by aqua fortis. The method was validated through the soil (GBW07401) and plant (GBW07603) for national standard matter. The relative error of its result is between 0.00% and 7.14%, and the relative standard deviation is between 0.87% and 5.25%. The method is quick, handy, saving reagent and completely digesting in dealing with plant's ash and soil sample, and the accuracy and precision of results are satisfying.