Abstract: The research, for the first time, isolated cloned AGPase small subunits cDNA sequence of gene 5' end from cassava, and successfully obtained cassava AGPase small subunit gene key enzymes of starch synthesis in full length cDNA sequence. The study used cassava as test materials, set up anchor primers, and designed specific PCR primers according to the known sequences of cassava AGPase small subunit gene. Then, the study used reverse transcription primers AUP1 retroviruses total RNA, separated and purified the reverse transcription product, and based on the improvement of three enzymatic reaction (the reverse transcription reaction, the TdT and tail, nested and touchdown PCR technology), to carry out the comparison of the new method with the traditional one. Using the modified method, the cassava AGPase key enzymes of starch synthesis in the small subunit gene 5' end cDNA sequence was obtained successfully for the first time. Meanwhile, compared with the traditional method, the improved RACE technology was much simple and fast in operation，more efficient in obtaining sequence and cheaper in test cost. Therefore, this study can be efficient and quick to obtain cassava starch key enzyme AGPase small subunit gene 5' end sequences, and had certain practicability and simplicity.