The vital sign, including respiration and heartbeat signal, can be detected through a non-contact Doppler radar. Since the harmonics and intermodulation products of the respiration are major interferences according to the previous researches, the heartbeat signal cannot be clearly identified without holding the breath, detected from back of the subject or just filtered to remove respiration signal. A synchronized motion technique (SMT) based on Doppler concept is proposed to suppress the interference from the respiration in non-contact heartbeat detection. The experiment results show that the SMT effectively improve the heartbeat-to-respiration ratio (HRR) up to 76 % while subject kept normal breathing. The SMT mechanism is also analyzed through non-linear simulation which agrees well with the measurement data.