In this paper

the intrinsic qualities of invariant terms in 3D MLFMA are discussed at first.They are

α

m

l

m'

l

's invariance of translation

the central conjugate symmetry of

V

s

or

V

f

on

space

and the symmetric sparse matrix

sparse

and equivalency between

V

s

and

V

f

while using Galerking method.Using these factors

we can optimize the invatriant terms' calculation in MLFMA program.As a result

the complexity of calculating α

m

l

m'

l

is reduced from

O(M

l

(6

3

-3

3

))

to

O(7

3

-3

3

)

and even to

O

((7

3

-3

3

)/8)

V

s

and

V

f

are reduced from

O(K

L

N)

to

O(K

L

N/4)

and

A

ji

from

O(N)

to

O(N/2)

.Numerical results show the validity of optimizing.