Covert channel is a famous drawback exists in most of multilevel security systems. Both TESEC and CC standards need covert channel analysis when secure software tries to get the certification of some security levels, i.e. B2 and EAL5 or above in TCSEC and CC, respectively. Search method is one of the most important works with ad hoc characters in covert channels analysis. Though some semi auto tools have been built, peoples who work in this area are eager to develop an auto search tool to find all of covert channels since it was first known in 1973. This paper proves that willingness is a kind of undecidable problems, by which illustrates it's impossible to build a program which can identify all of covert channels in a security computer system automatically.