The invention relates to the topographic technique field, and provides a ranging method and a system, wherein the ranging method comprises the following steps: modulating measured beam, by a modulated signal of which the frequency is continuously changed over time; emitting the modulated measured beam to a measured object; receiving the reflected measured beam and converting the beam into the measured signals; and comparing the frequency of the modulated signal generated when the measured beam is received with the frequency of the measured signals, thus determining the measured distance.