In order to identify the number of rotor blades and rotation period parameters of low-altitude multi-rotor UAVsa method of parameter estimation for multi-rotor UAVs based on ISAR sequences was proposed.Euler rotation matrix was used to build a motion model for multi-rotor UAVsand the ISAR imaging mechanism of multi-rotor UAVs was analyzed.Then the ISAR sequence of multi-rotor UAVs was obtained by using the range-instantaneous Doppler algorithm.Through the imaging simulationit was discovered that strong scattering would appear on the imaging surface when the rotor is perpendicular to the radars visual direction.The ISAR data was pre-processedand the distribution matrix of strong scattering in the time-range domain was obtained.Then the number of rotor blades was estimated by using the ant colony algorithmand the rotation period was estimated by autocorrelation.The simulation results show that the proposed method has a good anti-noise performance.The number of rotor blades and the rotation period could be estimated when the SNR was greater than -10 dBand the estimation error of the rotation period was less than 8%.