The invention discloses a phase-locked loop frequency synthesizer with a loop gain calibration function, including a phase-lock loop and a gain calibration unit. The phase-lock loop is composed of a frequency/phase discriminator, a charge pump, a loop filter, a voltage-controlled oscillator, and a frequency divider; the gain calibration unit is composed of a state machine, a comparator, two analog-digital converter, a reproductive charge pump current source, and a reproductive loop filter resistance R. the invention implements an automatic correction of the loop gain or bandwidth of the frequency synthesizer, the correction is dynamical, elements such as technique, temperature, voltage, longevity and the like are all corrected automatically, its unit structure is simple, a power consumption is low, and the invention is specially suitable for the wireless radio frequency requiring a stable loop gain.