To realize nonparametric imaging of targets in complex micromotion, the available method achieves scattering center trajectory association by Kalman filtering with the minimum Euclidean distance criterion. Then, it solves the 3-D distribution of scattering centers using matrix factorization. For targets with complex structure, however, the Kalman filter usually leads to wrong association due to serious trajectory intersection in the range-slow time domain. To tackle this problem, this paper proposes an effective trajectory association method based on the 2-D range-instantaneous-Doppler image series. Additionally, it refines the trajectory matrix using modern spectral analysis to facilitate accurate 3-D imaging. Simulation results have proved the effectiveness of the proposed method.