We propose a novel atomic beamsplitter for guided atoms composed of a U-shaped current carrying conductor and an additional bias magnetic field, and study the dynamic process of atomic beam splitting in the beamsplitter by Monte Carlo simulations. Our study shows that the splitting ratio of the beamsplitter can be continuously adjusted from 0 to 1by changing the additional bias field. In addition, the transverse temperature of guided atoms at the outlets of the beamsplitter are also estimated and explained qualitatively.