A line readout circuit with digital outputs is presented in this paper, which is suitable for SOI diode uncooled infrared focal plane arrays. The readout channel consists of a second-order sigma-delta modulator and an output buffer. The modulator includes an integrator, a current DAC, and a comparator. With the proposed circuit desiqn which adopts direct digital guantification, traditional analog pre-amplifier circuits are no longer needed. Continuous time increment type sigma-delta modulator can be driven by weak signals, which is capable of dealing with relatively small signals outputted by SOI diodes. The readout circuit was designed and simulated with 5 V supply voltage and the readout channel width was limited below 30 μm to meet the requirement of pixel physical size. The tape-out was completed in SMIC 0.35 μm CMOS technology to further validate the effectiveness of this design. Test results show that the output noise was less than 9 μV with 60 Frame/s under room temperature, the power consumption of each channel was less than 450 μW with 5 V power supply voltage, and the area of each channel was 0.092 mm² . Compared with traditional readout circuits, the proposed circuit needs no pre-amplifier circuits nor off-chip ADC, which reduces the complexity of the system. This design can be used in low power, low noise, and high density large scale infrared imaging systems with less chip area and power consumption.