The present disclosure provides a method and a device for controlling a downlink optical signal in a passive optical network, and a computer-readable storage medium. The method includes: monitoring power of a downlink optical signal in a process of receiving the downlink optical signal; and adjusting an attenuation value of a variable optical attenuator at an ONU side according to the power of the downlink optical signal until the power of the downlink optical signal falls within a preset power range. The variable optical attenuator at the ONU side is located between an optical splitter and an ONU.