As xml becomes a standard for disseminating data on the internet, applications disseminating xml data on the internet are rapidly increasing ,so flow of xml data on the internet could breach the privacy of data providers unless access to the disseminated xml data is carefully controlled. the methods using encryption have been proposed for such access control .dna cryptography is a new born cryptographic field emerged with the research of dna computing. the vast parallelism, exceptional energy efficiency and extraordinary information inherent in dna molecules are being explored for computing, data storage and cryptography. we briefly introduce the biological background of dna cryptography and the principle of dna computing and propose a novel encryption algorithm is based on number conversion,dna digital coding,and use of enzymes , which can effectively prevent attack on xml file and provide excellent security.