The invention relates to a method to prevent user data stored in personal digital assistant device from being lost. The personal digital assistant device has a nonvolatile read/write memory, a RAM anda battery. Where, the nonvolatile read/write memory has a presetting region and the RAM stores user data and system state. When the electricity quantity in the battery is less than a given value, itwrites the user data and system state into the presetting region. When the personal assistant device is resupplied with power, it stores them back into the RAM.