Abstract: Optical dead-path is a major error source in the laser interferometer, which, however, is hard to be accurately measured. Joule balance is a mass measurement apparatus traceable to the Planck constant, in which the laser interferometer is used to measure the relative displacement between the suspended coil and the exciting magnet, while the optical dead-path in the interferometer is so large that displacement measurement in the air has been adversely affected. Given this problem, this paper discussed an optical dead-path measurement method based on the optical path difference measurement between the vacuum and non-vacuum environment. This method uses a vacuum system to change the air pressure where the measurement optical path is located, and measures the optical range difference introduced by the change of the refractive index of air, and then calculates the length of the optical dead-path between the excitation interferometric path and the suspension coil interferometric path. The method can reduce the optical dead-path measurement uncertainty from millimeter to micron scale. In addition, by using the dead-path as the absolute distance, this paper discussed the vertical relative position measurement between the suspended coil and the exciting magnet, so that the relative zero position is measured.