Conjugated polymers are polymer with semiconductor properties due to the π-π conjugation structure in its main chain. Conjugated polymers are promising material that has garnered significant interest for various applications such as organic solar cells, organic transistors. The molecular weight of conjugated polymers is recognized as one of the most significant factors influencing the morphology of active layer and the performance of the device. And there exists a specific molecular weight that yields the highest performance. However, the traditional batch system results in batch-to-batch variations, making it challenging to control the molecular weight of conjugated polymers. On the other hand, the continuous flow system offers several advantages such as a larger reaction surface area per unit volume and efficient heat and mass transfer, which leads to shorter reaction times and high reproducibility in synthesis. In this study, we used our continuous flow system to synthesize PM7 D1 and investigated the factors influencing its molecular weight. Finally, we optimized the reaction condition to control the molecular weight of product and successfully synthesized PM7 D1 with specific molecular weights of 30 kDa, 40 kDa, and 50 kDa with high reproducibility.