Abstract: A high integration density front-end readout circuit(ROIC) was presented and designed in allusion to the kind of light laser detection and ranging(LiDAR) using avalanche photodiode(APD) detector. The front-end readout circuit consisted of high-gain broadband transimpedance amplifier (TIA), fixed gain voltage amplifier, high-speed real-time comparator, low voltage differential signaling (LVDS)output interface et al. TIA adopted resistance feedback structure. The output voltage signal of TIA connected to next stage through the way of pseudo-difference signaling, therefore obtaining better anti-interference ability. Fixed gain voltage amplifier was a non-inverting amplifier with resistance feedback network. Then a decompensated open-loop amplifier was used for the comparator in the chip. The ROIC can connect with FPGA directly profiting from the LVDS, again improving the integration density of the chip. The circuit was designed with chrt 0.18 m process. The area of chip was about 0.9 mm1.9 mm. The voltage of the power supply to the chip was 3.3 V. Through the test, the gain of TIA can achieve 83.6 dB, the bandwidth can reach 120 MHz. At the same time, the whole readout circuit can make an immediate response for input pulse signal with 5 ns pulse width. The LVDS which was designed as the output of whole readout circuit can meet the request of FPGA's interface.