The basis to predict earthquakes by the double magnetic storm time method is that the crustal expansion and contraction caused by the migration of strain energy present a cycle with a double time relation.The Tangshan M_S7.8 earthquake was the strongest one in 1970s in our country,it had a relation to four groups of magnetic storm energy based on the double magnetic storm time method.Every group of magnetic storm was headed or followed by earthquakes.When the residual energy accumulating after these earthquakes,the crust expanded and contracted,the geomagnetic field was affected and the magnetic storm formed.So the magnetic storm has a relation to variation of earthquake energy and earthquakes can be predicted by the double magnetic storm time method.