We report an improvement to the lateral-magnetic-tweezers which can be used to stretch and twist DNA molecules, and to probe the interaction between DNA and proteins. The apparatus enables not only direct measurement of the extension and rotation of DNA but also easy refreshing of the buffer in the flow chamber. With this apparatus, the experimental efficiency can be greatly improved by manipulating several DNA molecules simultaneously. The force exerted on DNA molecules ranges from 0.1 to 40pN, satisfying the requirement of most single molecule experiments. To demonstrate the applicability of the apparatus, we measured the extension-force curve and the extension-rotation curve of DNA and studied the translocation of DNA by a motor protein. The results agree well with those reported in the literature.