A new method for measuring the relative attitude of a load on an airborne photoelectric platform supported by three-axis universal joint structure is proposed. The mathematical model for angle measurement is established, based on which the linear relation between the angle and distance is derived by using the eccentric method. Furthermore, the relative attitude measurement method for a load rotating along the yaw and pitch directions is also proposed based on a unique eccentric structure. Finally, to validate the proposed new method, a comprehensive experiment for measurement angle from -5° to +5° in increments of 0.5° was performed by using the angular capacitive sensor. The experimental results show that the precision of the angular measurement is better than 15″ with maximum 1.24 kHz bandwidth, which meets the relative attitude measurement requirements of the internal load on the airborne photoelectric platform.