Biomagnetic fields produced by organisms carry valuable informations of electrophysiologies and pathologies, which are commonly detected by superconducting quantum interference device. As an ultra-sensitive magnetic field detector with the sensitivity of femtotesla level, the ultra-sensitive atomic magnetometer plays an important role in detection and study of biomagnetism. We introduce the sources and characteristics of biological magnetic field signals, as well as the physical mechanism and classification of ultra-sensitive atomic magnetometers. We also summarize and forecast the applications of ultra-sensitive atomic magnetometers in the field of the biomagnetic field detection, such as magnetocardiography, magnetoencephalography, and neurosciences.