The invention discloses a method and a device for improving extraction effect of prewashed methanol during low-temperature methanol washing and belongs to the technical field of organic chemical engineering. The method includes: using a prewashed methanol storage tank A and a prewashed methanol storage tank B which are idle as extractors, sending unqualified crude benzene in a crude benzene storage tank into the prewashed methanol storage tank A, feeding bottom water through a prewashed methanol storage tank pump, a second view mirror, a first stop valve and a third stop valve into the prewashed methanol storage tank B after sufficient standing for separating water in crude benzene, observing color of a medium at a pump outlet through the second view mirror, opening a sixth stop valve to send qualified crude benzene out of a boundary area when crude benzene appears in the view mirror; after all crude benzene is sent out, feeding methanol water in the prewashed methanol storage tank B to the extractors through the prewashed methanol storage tank pump, the second view mirror and a fifth stop valve to go back into a system. The prewashed methanol storage tanks which are idle are utilized as the extractors, the advantage that equipment is high in capacity and long in dwell time is utilized, and sufficient separation of crude benzene and methanol water is realized.