The modified layer of the device dielectric layer on the Si/SiO2 substrate was prepared using polymethyl methacrylate(PMMA), which improved the interface quality of the dielectric layer and induced the growth of the active layer, thereby increasing the crystallinity of the active layer.The organic thin film transistor (OTFT) was prepared by vacuum evaporation of a pentacene/rubrene double layer active layer, and the effect of device performance on the thickness of the rubrene layer was investigated. The test results show that the threshold voltage of the modified device is -3.55 V, the current-switching ratio is more than 105, the mobility reaches 0.0558 cm2/V?s, and the subthreshold swing is 1.95 V/dec. These data suggested that the overall performance of devices with the modification of PMMA layer was successfully improved.