The invention discloses a relative measurement method for aircrafts. The method comprises: GNSS observation data of two aircrafts are obtained; according to the GNSS observation data, respective track dynamical models of formation flight of the aircrafts are determined and single difference observation equations of the aircrafts are constructed; on the basis of the respective track dynamical models of the two aircrafts, a relative track dynamical model between the two aircrafts is determined; and filtering estimation is carried out on the relative track dynamical model, the relative state, and the single difference observation equations, a relative state between the aircrafts is determined. Meanwhile, the invention also discloses a relative measurement system for aircrafts. With the method and system, measurement of a relative state between aircrafts can be realized.