Here, we review the progresses of researches on the biological effects of the hypomagnetic field (HMF) and propose that a magnetic field with magnetic induction of "0 < |B| ≤ 5 μT" is defined as a hypomagnetic field (HMF)．Interplanetary space is a natural hypomagnetic field．The astronauts, enabled by space sciences thriving in the recent decades, are spending longer time in the hypomagnetic outer space, e.g．landing on the moon and heading to mars. The effects of HMF on many aspects of biological processes, especially the adverse impacts on the functions of the central nervous system, remind us that the astronauts would suffer from potential risks due to the HMF exposure．Unfolding the mechanism of the biological responses to the HMF is the fundament for developing the counteractions of the adverse space environmental factors, and has recently become a hot topic in the field of space life sciences. Refining the concept of HMF and standardizing the HMF simulation systems for biological experiments will benefit the comparability of the HMF effects as described, and improve our understanding of how HMF influences homeostasis, cell signaling pathways and cognitive behaviors, and to what extent HMF contribute to such disturbances.