Stepped frequency continuous wave (SFCW) radar is a promising noncontact method to detection of human vital signs. Antenna as a key component of radar directly related to the performance of the radar. In this letter, we present a 0.5‐1.2 GHz compact multiple cut‐corners square spiral antenna for SFCW radar system. Compared with conventional planar spiral antennas, the improved antenna possesses the advantages of miniaturization and excellent axial ratio. The gradient‐change of the cut‐corners along the radiation determines its operation range: large cut‐corner corresponds to high frequency, while small cut‐corner affects low frequency. This design successfully leads to a small geometric size of 0.282 × 0.282 × 0.133 m. We measured this improved antenna in an anechoic chamber, and the results demonstrated that the circular polarization bandwidth for axial ratio ≤3 dB approached 100%, and a right‐hand circularly polarization (RHCP) gain of 4‐8.98 dBic in end‐fire direction is achieved in its operational range. Finally, we designed the experiment, experimental results show that the proposed antenna can work well with the SFCW radar in the detection of human vital sign (respiratory signals), indicating a great potential for detected life in practice.