Low-dose x-ray computed tomography (CT) is clinically desired. However, the quality of low-dose CT image is severely degraded due to excessive photon quantum noise and electronic noise. It is known that accurate noise modeling is a fundamental issue for low-dose CT imaging, such as statistical iterative image reconstruction and statistics-based singoram restoration. In this paper, we first studied the statistical moment properties of the noise model in CT projection domain wherein the noise of detected signal is considered as quantum photon noise plus background electronic noise. More importantly, we derived a new formula to estimate the mean-variance relationship in Radon domain by using Taylor explanation. To test the presented variance estimation formula, an anthropomorphic torso phantom was scanned repeated by a commercial scanner at five different mAs levels from 100 down to 17. The experimental results demonstrate that the electronic noise is significant when low-dose scan is performed or the number of detected photons is limited. As an important conclusion of the presented study, electronic noise effect should be considered in low-dose CT image reconstruction.