Because of coast diurnal station beyond control area and seabed diurnal station difficult to place and recover with low security guaranty, marine magnetic survey far beyond the sea is faced with a challenge of diurnal correction problem. The principle of marine magnetic gradient survey and its data characteristics are researched, and a new concept termed as simultaneous survey point is first presented, and a set of precise mathematic model are accordingly produced to calculate diurnal correction alone every survey line.