The current infrared detectors are difficult to achieve their own dynamic range by primary integration using traditional readout methods. In order to achieve large dynamic range readout of infrared detectors without changing gears， this paper introduces Pulse Frequency Modulation （PFM） and adopts the CTIA input stage structure to ensure the injection efficiency when the signal is weak， and investigates the method of large dynamic range readout of infrared detectors without changing gears. A CTIA input-level pulse frequency modulation （PFM） readout method was proposed to build an experimental system at the system level and conducted digital quantization experiments with short-wave infrared InGaAs cell detectors. The conversion linearity problem caused by the delay time of system structure at strong signal was analyzed in detail， and a digital quantization conversion model under non-ideal conditions was established. The experimental results show that the dynamic range of the proposed CTIA input stage PFM IR detector readout method reaches 97 dB， which provides a feasible solution for large dynamic range readout of IR detectors without changing gears， and lays a theoretical foundation for digital readout circuit design.