1. A direct current (DC) compensation method for S-parameter rational functions, the method comprising: reading S-parameters f(sk) which are generated at given frequency sk from a storage unit, the S-parameters indicative of an n-port network; supplementing an S-parameter, which is generated at frequency sk=0, into the S-parameters f(sk) using an interpolation method, upon the condition that there is no S-parameter which is generated at the frequency sk=0; generating an S-parameter ration function according to the S-parameters f(sk) using a vector fitting algorithm; compensating a DC level of the S-parameter rational function to generate a compensated S-parameter rational function; and storing the compensated S-parameter rational function into the storage unit.