The invention relates to the technical field of communications. Disclosed in an embodiment of the invention are a circuit and method for acquiring PAM4 decoding thresholds and an optical network unit for increasing accuracy of decoding thresholds, and reducing complexity in realizing a circuit. The decoding thresholds comprise Vth1, Vth2 and Vth3, and Vth1, Vth2 and Vth3 satisfy the following condition: Vth1 < Vth2 < Vth3. The circuit comprises a mean value module and a microprocessor. After determining that a signal inputted to the mean value module is a target signal, the microprocessor collects a target decoding threshold obtained by performing, by the mean value module, mean value computation on received voltages of symbols in the target signal. When the target signal is a first signal, the target decoding threshold is Vth1, and the first signal comprises a symbol representing 00 and a symbol representing 01; and/or when the target signal is a third signal, the target decoding threshold is Vth3, and the t
