A method for realizing acquisition of on - line information of user includes obtaining cipher key living period from user identification module when set - top box is switched on ; issuing certification request to front end by set - top box if living period is expired ; carrying out certification by front end and setting user terminal as on-line state as well as sending response information including new cipher key data back if certification is passed ; updating cipher key data by set - top box when it receives response information of certification success .