Respiratory rate is an important vital sign that reflects health status. In hospital, traditional methods of measuring respiratory rate include manual counting and using a respiratory belt, both of which may pose a risk of contact infection. This study conducted clinical trial in medical ward of hospital, presented a non-contact method based on a 24 GHz continuous-wave radar to detect the displacement of the chest wall during breathing. By processing the radar signal, we were able to obtain the respiratory rate. However, during clinical validation, uncertainty in the daily measurement time of vital sign, which serve as the gold standard, can lead to comparison error. To overcome the interference caused by the issue, this study proposed a validation method that calculates the average of radar-detected respiratory rate over specific time interval and uses the standard deviation of these time interval as an indicator of the stability of the radar-detected respiratory rate value. Statistical results show that the radar-detected respiratory rate derived in this study is consistent with the hospital-measured respiratory rate (p = 0.17) and exhibits significant differences across different subjects (p < .05). The standard deviation of the radar-detected respiratory rate varies across different time. These findings confirm the feasibility of replacing hospital-measured respiratory rate with radar-detected respiratory rate. Using this non-contact method to detect respiratory rate can reduce the risk of contact infection. Moreover, the stability indicator of radar-detected respiratory rate proposed in this study can provide healthcare personnel with a reference for assessing the stability of a patient’s respiratory status over a specific period of time.