Formation flying has emerged as a prominent aircraft operational mode， necessitating precise relative positioning of aircraft within the formation as a fundamental requirement. In Global Navigation Satellite System （GNSS）-denied environments， aircraft experience a loss of global positioning information， making it challenging to derive relative aircraft positioning solely from local data sources. To address the relative positioning challenge among aircraft within a formation in GNSS-denied environments， this paper introduces a method that integrates data link and inertial navigation. Firstly， the inertial navigation method is employed to give a real-time calculation of the positioning information of each aircraft， which is then transmitted to other aircraft through its respective data link. Subsequently， each aircraft conducts measurements of the relative positions between the aircraft in the formation by using the received information from other aircraft and the data link. Finally， leveraging the continuous time series of inertial navigation data and data link measurements， a relative pose optimization factor graph is constructed to solve the relative poses between the aircraft in real time. Simulation and experimental verification are undertaken， employing a two-aircraft formation as a case study. The outcomes indicate that this method proposed enables real-time estimation of the relative positions of aircraft within the formation. Experimental results demonstrate that this method reduces the distance error measured by the data link by 76%， and can provide accurate and reliable relative position information for formation flight.