A pulse modulation method for adjusting pulse signals according to a working voltage of a load, the method includes the following steps. The working voltage of the load is detected by a feedback terminal of a PWM controller. Whether the load is a light load or a heavy load is determined, if the load is a light load, a pulse skipping mode is entered; the voltage output to the load is detected by the feedback terminal. Whether the voltage output to the load is normal is determined, if the voltage output to the load is low, the pulse skipping mode is stayed for a number of clock cycles; the voltage output to the load is detected to determine whether the voltage is normal. If the voltage output to the load is still low, the pulse skipping mode is jumped out and a normal working mode is entered.