The properties of strongly interacting matter in the external magnetic field have attracted many studies in recent years as strong magnetic fields appear in heavy-ion collisions, the early universe, and magnetars. Many novel and unexpected pheonmena have been found from lattice QCD studies, such as inverse magnetic catalysis, reduction of chiral transition temperature in strong magnetic fields, and a plasuible QCD critical end point in the plane of temperature and magnetic field. In this talk I will present the first latiice QCD study on the Gell-Mann-Oakes-Renner (GMOR) relation in the external magnetic field, and find that the GMOR relation can naturally reconcile the magnetic catalysis at zero temperature and reduction of chiral transition temperature in nonzero magnetic fields. I will further dicuss the change of the degrees of freedom and the strength of transition in the strong magnetic fields via fluctuations of net baryon number, electric charge, and strangeness, and propose certain observables to detect the existence of a magnetic field in the late stage of heavy ion collisions. The talk is based on arXiv:
2008.00493 and 2104.06843.