The utility model provides a volume auto -adjusting circuit of on -vehicle radio and tape player, include: radiofrequency signal receiving element, filtering and amplifying circuit, voltage comparater, fader and volume adjustment module. Filtering and amplifying circuit's input links to each other with radiofrequency signal receiving element's output, and filtering and amplifying circuit's outputlinks to each other with voltage comparater's first input end, and fader's first output links to each other with voltage comparater's second input, and voltage comparater's output links to each otherwith fader's input, and fader's second output links to each other with radiofrequency signal 0's control end. Radiofrequency signal receiving element is used for receiving the radiofrequency signal that cell -phone switch -on back produced in the certain limit, and according to radiofrequency signal produces induced -current. When induced voltage that induced -current produced is greater than thefirst voltage of fader output, close the audio output of on -vehicle radio and tape player. The utility model discloses can reduce the safe risk that the vehicle went.