thanks. i have found out the real reason for this problem and have solve it.

i miss conclude this problem due to
power supply,in fact it is due to reset signal is controlled by a supervisor chip,
and this chip can be controlled by rs232 line,so when computer power on/off,it will toggle rs232 voltage level,then cause my system to be reset.
when i disconnect the serial port from the computer,this problem never comes again.

anyway.thanks all who concern my problem and give me good suggestion.