A system and a method of diagnosing a state of a low-voltage battery, in which, in order to diagnose the state of a low-voltage battery (12V aux battery) provided at a low-voltage side, a daisy chain circuit unit receives a voltage from the low-voltage battery and outputs one or more result signals, and an isolator unit, which electrically insulates a high-voltage side and the low-voltage side, converts the outputted one or more result signals and provides the converted signals to a control unit, thereby diagnosing a state of the low-voltage battery at the low-voltage side by using the control unit provided at the high-voltage side.