When disturbances such as sudden load changes，generator tripping，or transmission line faults occur，the system will experience a power swing （PS）.In this case，the impedance trajectory detected by the distance protection may encroach into zone Ⅲ of the protection，leading to maloperation or refusal operation of the distance protection and triggering cascade failures.Therefore，it is crucial to accurately distinguish PS and faults.A distance protection method based on improved ellipse fitting is proposed.Firstly，a Lissajous figure （LF） is constructed by using voltage and current signals at the protection mounting location，and a critical condition detection algorithm （CCDA） based on this figure is implemented.Then，the elliptical area and its Hausdorff distance from the fitted signal trajectory are calculated to distinguish the critical conditions and faults of the system.Next，the critical conditions such as various PSs，load intrusion，voltage instability，and faults at the electrical center are simulated and analyzed.The influence of factors including fault location，fault resistance，fault type，and inception angle are also analyzed.Finally，by simulating the case studies，it is demonstrated that the proposed algorithm is applicable to various complex critical conditions and accurately fits signal trajectories；the tripping is avoided even when the observed impedance trajectory intrudes into zone III of the protection；the functions of PS blocking and de-blocking are rapidly recognized and executed.The research results show that the method does not rely on high sampling rates，exhibits easy calculation and good noise immunity，and maintains fast response speed，effectively distinguishing remote PSs and high-resistance faults.