The invention discloses an all-fiber laser frequency measuring device and method and belongs to the field of femtosecond laser precision measurement. The device comprises a laser to be measured, a fiber femtosecond laser frequency comb, a microwave atomic clock, a fiber beam splitter, a fiber beam combiner, a fiber looper, a tunable fiber grating, a fiber input avalanche photodetector, a fiber input wavelength meter, and a frequency counter. The method comprises the following steps of: the laser to be measured and a frequency comb laser are transmitted through a fiber and are subjected to beat frequency by the fiber beam combiner; tuning the tunable fiber grating to select out a frequency band generating the beat frequency, entering the fiber input avalanche photodetector via the fiber looper to obtain the beat frequency; and obtaining the accurate frequency value of the laser to be measured by using repetition frequency, carrier envelope offset frequency, beat frequency, and a wavelength meter coarsely measured frequency. The device and method can avoid the adjustment of a space optical path, improve a signal-to-noise ratio, are easy to integrate, do not need to repeatedly replace components and adjust the optical path, and have simple structure, convenient operation and a wide measuring range.