An over current protecting circuit, used with a switching converter having a switch, having: a current limiting circuit, configured to receive a limit indicating signal and to provide an on time signal to control an on time period of the switch based on the limit indicating signal, wherein if the limit indicating signal indicates a current flowing through the switch of the switching converter is larger than a limiting threshold, the on time period of a next switching cycle of the switch is subtracted with a first period, otherwise, the on time period of the switch of the next switching cycle is added with a second period; wherein a maximum on time period of the switch is fixed to be an on time reference.