In this Letter, we present a method to measure the eccentricity of an off-axis asphere using a laser tracker during optical null testing. We first adjust the optical path of the null testing, and then probe some necessary reference surface on the compensator or the off-axis mirror’s body with a laser tracker. Next, using the collected data to process, construct, build the coordinate system, solve, and so on, we obtain the eccentricity directly through comparing the central point of the asphere and the tested optical axis. A measurement experiment is conducted with a circular off-axis aspherical mirror; the result shows that the measurement accuracy can reach 0.1859 mm, 6.2% of its tolerance belt.