The coupled angles between different axes of an Electrostatically Suspended Accelerometer (ESA) are related to the parallelism error of a sensor head, and it is hardly measured accurately by using classical methods. Therefore, a novel online measurement method for the coupling angles was proposed by a varying preload voltage. By modeling the relationship of the parallelism error, coupled angle, attitude angle and the electrostatic force coupled angle, the mechanism and equation of the varying preload voltage method for online measurement of the coupled angles were given. A ESA prototype was designed and then it was used to verify the proposed method. Experiment results show that the coupling angles of X/Y and X/Z are -5.10×10-4 rad and 2.36×10-5 rad, respectively, which are consistent with that of theoretical analysis. The method is also suitable for online measurement of the coupling angles for Y/X, Y/Z and Z/X, Z/Y. The result shows that this novel varying preload method can online identify the coupled angles of different axes of ESA effectively and accurately.