This paper presents an uncooled infrared imager with flicker noise reduction, which is realized by the proposed phase difference modulation (PDM) method. The PDM method makes noise transfer function generate a series of notches to reduce flicker noise. To realize PDM, a high-speed sub-frame readout circuit is proposed. The readout circuit has been manufactured and the output data is processed with FPGA. The measurement results show that the flicker noise power in the frequency range above 10Hz is reduced by 42% compared with traditional method at 60fps full image output rate, and the noise equivalent temperature difference (NETD) is reduced from 40mK to 34mK.