Abstract: High mass resolution spectrometer for isotope seperator on line(ISOL) in Beijing radioactive ion－beam facilities(BRIF) project requires mass resolving power of 20 000. A bi－energy system with big and small dipoles is used to reduce energy dispersion which is too large to get mass resolving power. When the width of beam with a Gaussian distribution is 0.06 mm and 10 mrad, resolving power of this spectrometer is 33 400. After consideration of 0.002% energy spread introduced by acceleration, the final resolving power of this spectrometer reaches 20 000.