The group refractive index of BK-7 glass material is accurately measured using a tandem low-coherence interferometer, which is composed of a Mychelson interferometer and a Fizeau interferometer, within a combined standard uncertainty of 8.4ppm. The experimental results are compared with the value calculated from the conventional data base on the phase refractive index, within a difference of about 4.7ppm. This new method is applicable to in situ measurement due to the principle of its common optical path.