The invention belongs to the technical field of wireless communication, and relates to a voltage-controlled oscillator (VCO) in a phase-locked loop, in particular to a Class-F voltage-controlled oscillator. The oscillator comprises a first resonant cavity, a cross coupling tube pair and a second resonant cavity, the first resonant cavity resonates at omega1, and the second resonant cavity resonates at 3omega1; that is to say, two separated inductors are adopted to form two resonant cavities, third harmonics and fundamental waves are separated, and a waveform similar to square waves is still formed at the drain electrode and the source electrode of the cross coupling tube. Moreover, the two resonant cavities replace a transformer structure, thereby avoiding repeated adjustment of the size of the transformer to ensure the performance requirements of the circuit, greatly simplifying the design process, and enabling the actual design to be more convenient. Meanwhile, a tail current sourceand a filter capacitor are removed, so that the circuit structure is simpler, and the problem of phase noise deterioration caused by the tail current source and the filter capacitor in the circuit iseffectively avoided.