Conical mirror is a preferred choice for fluorescence molecular tomography (FMT) because of its ability to collect fluorescent emission photons from the whole surface of the imaged object such as mice. Conical mirror, however, would lead to a fraction of photons to be reflected back to the mice surface, including excitation photons and emission photons, which result in inaccurate source positions and measurements errors in the FMT forward modeling and reconstruction. Based on Monte Carlo simulations, we have studied systematically the effects of multiple reflections of different conical mirror designs. We first generated a multiple reflected photon map for each design of the conical mirror, and then we applied Monte Carlo simulations to model photon propagation inside tissues. Finally, we evaluated the ratio of the multiple reflected photons to the total photons, and figured out the optimized size of the conical mirror. Our simulations demonstrated that a single conical mirror configuration could eliminate the multiple reflection issue while keep the imaging system setup simple when its small aperture radius is larger than 5 centimeters. We then fabricated a conical mirror with the optimized size according to the Monte Carlo simulation results, and performed phantom experiments with both the optimized conical mirror and the non-optimized one. Phantom experiment results show that noises in the reconstructed images are reduced with the optimized conical mirror, and the reconstruction accuracy is improved as well.