Problem: A micromotion period estimation based on the Viterbi algorithm and the timefrequency squared difference sequence is proposed. Procedure and method: The micromotion states of the radar target is dependent on the dynamics, structure of the target, and they are usually different for different targets. As a result, the micromotion states reflect some important information of the target and can be used to aid the target detection and recognition, which has a great importance. In the research of the micromotion features, space targets such as missile are hot points. Numerous micromotion feature extraction methods for space targets are published. Usually, the motion of the space target is compound of the translation and the micromotion. The implication of the translation on the micromotion feature extraction should be considered. This work focuses on the space target with compound motion. A micromotion period estimation based on the Viterbi algorithm and the timefrequency squared difference sequence is proposed, in which the Viterbi algorithm can provide robust instantaneous Doppler estimation in low signaltonoise ratio (SNR) and the timefrequency squared difference sequence can remove the effect of the translation. Thus, the proposed method has a good performance. Results: Numerical results prove the effectiveness of the proposed method.