  A general form of the dynamical equations of field is obtained on the
requirement this field is a superposable one; hence the constraint on the forms
of the Lagrangians is acquired. It shows this requirement requires the
continuous transformation group of the Lagrangians of field to be compact, and
that All Lagrangians of elementary particles, such as leptons, quarks, photons
and gluons, satisfy this requirement.The result of regarding this character as
a general property of physical field is discussed.
