Carbon nanotubes are regarded promising building blocks for the future nanoelectronics. High performance transistors and integrated circuits based on semiconducting CNT thin films, e.g., CMOS type 4-bit adder, CNT computer, flexible sensor etc., have been reported. These progresses demonstrate the CNT transistor technology is being pushed to practical application. For real circuits applications, high I_on/I_off ratio with low I_off for a transistor are required in high speed and low power consumption integrated circuits. However, the switching behavior of most reported CNT thin film transistors are not good enough. Due to the intrinsic ambipolarity of CNTs, the I_off values are usually higher than hundreds of nano Ampe and the I_on/I_off ratio is decreased to less than 10² at relatively high source/drain bias voltages (e.g. 1V). Though the switching behavior of the FETs based on individual CNTs can be improved by either special design of the device geometry or chemical doping, these methods do not work well in CNT thin film transistors. In this work, we improved the performance of CNT thin film transistors by combining the asymmetric gate design and chemical doping. In this way, the I_off was suppressed while the I_on was increased at the mean time. Thus the I_on/I_off ratio was increased by ~2 orders of magnitude when compared with normally designed top-gate device. Moreover, the devices showed better subthreshold swing (SS) and the threshold voltage (V_th) was closer to zero than normal devices. These improvements are benefit to more complicated circuits design, and our method is applicable for the emerging nanodevices.