Abstract: Transgenic process may be with some undesirable molecules, such as error-connected carrier, DNA fragments in the PCR process, vector without inserted DNA fragment, other recombinant DNA molecule and insert incorrect fragment. These will bring some troubles to transgenic cloned. Studies showed that some carrier may carry different resistance-detecting genes, and the require cloning and unwanted non-purpose cloning region could be separated through a suitable screening step to achieve the object of purified DNA. The test gene and its principle of three carriers were described.