The study of gold nanoparticles (GNPs) has been more attractive because of its application for cancer detection and therapy. X-ray fluorescence computed tomography (XFCT), which is sensitive to high Z elements such as gold, is a promising approach to determine the distribution of GNPs in vivo. Several XFCT imaging systems have been designed and developed in recent years. However, as an emission tomography, the limit of fluorescence yield and low detection efficiency brings high statistic noise. The interference of Compton scattering as well as the effect of absorption when photons crossing the phantom should also be considered in data acquisition and image reconstruction. These are the main problems that current XFCT imaging systems confront. In this paper, we present a design of XFCT system with fan beam X-ray source and two photon counting detectors for XFCT imaging and attenuation correction. One detector with pinhole collimation is placed perpendicular to the beam direction for fluorescence photon and scattering photon collection. The second detector is placed behind the phantom to obtain transmission CT imaging for attenuation correction. The main advantage of using two detectors is that the X-ray fluorescence signal and the attenuation map can be obtained in one time of scanning procedure. Numerical simulations are performed to study the feasibility of the system and its Sensitivity to GNPs.