a method of assembling multi - fragment dna molecule , the method including : 1 ) constructing a vector system including an acceptor vector a1 , a first donor vector b1 , and a second donor vector b2 , and each of the acceptor vector a1 , the second donor vector b2 , and the second donor vector b2 including a multiple cloning site ; 2 ) introducing a first target sequence l m to the first donor vector b1 , m being an odd number starting from 1 , and introducing a second target sequence l n to the second donor vector b2 , n being an even number starting from 2 ; and 3 ) repeating 2 ) to assemble multiple fragments to yield a multi - fragment dna molecule .