Abstract: Linear avalanche photodiode (APD) focal-plane infrared detectors have a wide range of applications. Coupling APD detectors with multiple-mode readout integrated circuits can achieve multimode detection within a limited pixel area, thereby enhancing the integration of the detection system. In this study, we designed an APD readout integrated circuit with four modes: infrared thermal imaging, gated 3D imaging, laser ranging, and asynchronous laser pulse detection. The input-stage circuits of the four modes were multiplexed. A Krummenacher structure was used to suppress the influence of background radiation, thereby expanding the detection range of photon flight time. An improved time-discrimination circuit was proposed to enhance distance measurement accuracy by reducing the time-discrimination error. The readout integrated circuit was designed using a 0.18 μm, 3.3 V complementary metal-oxide semiconductor (CMOS), with an array size of 128×128, a pixel center distance of 30 μm, and a maximum charge capacity of 3.74 Me^－. Simulation results show that in the laser ranging mode, at an integration capacitance of 13 fF and a background current of 1-150 nA, the background current response amplitude was ≤1.35 mV, which is much smaller than the response amplitude of 280 mV when the laser response current is 500 nA. The amplitude sensitivity of the asynchronous laser pulse detection mode was approximately 110 nA, and the pulse width sensitivity was approximately 4 ns. For laser pulses with a response of 150-500 nA, the time discrimination error of the improved time discrimination circuit was approximately 4 ns. The multimode multiplexed APD readout integrated circuit designed in this study has certain engineering application value.