Recently, people have become more concerned about life and health, and are paying more attention to the detection of their vital signs indicators. Non-contact detection of respiration rate(RR) and heart rate (HR) by using millimeter-wave (mmWave) radar is common. In this article, we propose a detection system based on multi-vital box fitting approximation using millimeter wave radar. On the basis of using different vital boxes signals with a certain period correlation, a new vital signal can be obtained by joint multi-boxes fitting approximation to a new vital signal, and taking measurements of respiration rate and heart rate. The experimental results show that the accuracy of the system using the multi-vital box vital signal fitting approximation improves about 3% for the detection of RR and about 2% for the detection of HR than the traditional single vital box system.