In high-energy laser systems such as inertial confinement fusion systems, large-aperture optical components are widely used and transmittance is an important parameter for characterizing them. However, studies pertaining to the transmittance measurement of large-aperture spherical mirrors are few. Therefore, this paper proposes a curvature-radius-compensation technology to investigate the transmittance measurement of large-aperture spherical mirrors. First, the measurement scheme of a dual-beam single detector was adopted using the optical-power-ratio method. Second, to adapt to spherical mirrors with different curvature radii, a mathematical analytical solution was established to preliminarily adjust the angle and position of the mirror, compensate for curvature radius deviation, and finely adjust the curvature radius based on the displacement of light spot detected by the monitoring device. Subsequently, the compensation effect and factors affecting transmittance when measuring spherical mirrors with different curvatures were simulated and analyzed. The final simulation results show that proposed system can measure the transmittance of spherical mirrors with an effective aperture of less than 600 mm and a curvature radius of 1200 mm to ∞ at a measurement accuracy of 0.2%, thus satisfying the measurement requirements.