The disclosure provides a secondary light assembly, a night light assembly, a light fixture, and a method and unit for controlling the secondary light assembly. The secondary light assembly can be mounted on a main light assembly of the light fixture. The main light assembly is configured to provide first lighting light. The secondary light assembly includes: light emitting devices configured to provide second lighting light, a main sensor configured to detect the first lighting light of the main light assembly and generate a lighting light detection signal according to a detection result; and a control part configured to receive the lighting light detection signal from the main sensor and control the light emitting devices of the secondary light assembly at least based on the lighting light detection signal.