  If there is a null gradient field in 1+3 dimensional space-time, we can set
up a kind of light-cone coordinate system in the space-time. In such coordinate
system, the metric takes a simple form, which is much helpful for simplifying
and solving the Einstein's field equation. This light-cone coordinate system
has wonderful properties and has been widely used in astrophysics to calculate
parameters. In this paper, we give a detailed discussion for the structure of
space-time with light-cone coordinate system. We derive the conditions for
existence of such coordinate system, and show how to construct the light-cone
coordinate system from usual ones, then explain their geometrical and physical
meanings by examples.
