The present invention provides a method for cooling a CPU. The method includes the steps of: setting a passive cooling mode; setting a critical temperature and a reference temperature that is lower than the critical temperature; setting a plurality of temperature intervals between the reference temperature and the critical temperature; predetermining a duty cycle corresponding to each temperature interval; detecting a current temperature of the CPU; comparing the current temperature with the reference temperature and the critical temperature; entering into the passive cooling mode if the current temperature is between the reference temperature and the critical temperature; confirming which temperature interval the current temperature falls in; confirming a corresponding predetermined duty cycle according to the temperature interval; and switching the CPU between a working status and a sleeping status according to the confirmed predetermined duty cycle. A related system is also disclosed.