The invention provides a control circuit, a control method and a switching power supply. A frequency for controlling a conduction signal conducted by a main switch tube is a preset frequency when a switch-off time required by the main switch tube is larger than minimum switch-off time and is a frequency positively related to the switch-off time required by the main switch tube when the switch-off time required by the main switch tube is smaller than the minimum switch-off time, so that smooth switching between a fixed-frequency working mode and a variable frequency fixed switch-off working mode can be achieved, a mode switching signal is provided without a special mode judgment circuit, the control circuit is simple in implementation mode, and the expansion of a duty ratio of a switch control signal of the main switch tube can be stably and continuously achieved; and moreover, during the frequency conversion process, the power supply is still in the constant switch-off time working mode, so that the problem of subharmonic oscillation caused by insufficient subharmonic compensation due to frequency conversion is prevented.