A frequency divider circuit and a frequency synthesizer circuit are presented, comprising: first and second flip-flops; a phase inverter, wherein an output electrode of the first flip-flop is connected to an input electrode of the second flip-flop and an output electrode of the phase inverter, an output electrode of the second flip-flop is connected to an input electrode of the phase inverter and an input electrode of the first flip-flop, a control electrode of the phase inverter is connected to a control signal; and a control module, wherein the first flip-flop is connected to a voltage source through the control module, the control module is connected to the control signal and controls the connection between the first flip-flop and the voltage source. When the control signal is a first-mode signal, the first flip-flop is disconnected from the voltage source, providing a functionality of a N-division frequency divider. When both the control signal and an output signal of the second flip-flop are a second-mode signal, a functionality of a N+1-division frequency divider is provided.