In this work, we established a cascade cooling and two-step feeding technology for the energy-saving and fast synthesis of polyvinyl butyral (PVB) with a low molecular weight in a microreactor system. Because of the strong mixing capacity of the microreactor and the relatively higher initial reaction temperature of 60 degrees C compared to existing methods, the condensation reaction between the raw materials of poly(vinyl alcohol) (PVA) and n-butanal was highly intensified and the reaction rate was faster, along with less energy consumption for cooling. By reducing the aging temperature to 40-55 degrees C, which is far below the glass transition temperature of PVB, we could also lower energy consumption and improve the morphology of PVB. Benefiting from the new technology, the acetalization degree (AD) of PVB reached more than 78%. By applying the two-step n-butanal feeding method in the new technology, the utilization ratio of n-butanal was dramatically increased to more than 99.7%, which was the best result among current published reports. Moreover, a PVB product with a low molecular weight has been successfully prepared with the new technology. This work demonstrates a promising continuous microreaction process for greener and energy-saving synthesis of PVB.