A phase shift full bridge power converting system and a control method has the phase shift full bridge power converting system with a power converter, a controller, a load state detecting module, a switching controller, a switch module, a command generating module, a comparator, and a modulator module. The load state detecting module interfaces with the power converter and the switching controller. The load state detects module detects load parameter data of the power converter and switches a state at an output by using the switching controller. The controller obtains an error value between an output voltage of the comparator and a command voltage and calculates a duty cycle parameter by using the error value. According to the duty cycle parameter, a pulse control signal is generated to be outputted to a drive module for controlling the driving of the power converter.