Abstract: A Faraday cup is an absolute measuring equipment used to measure the accelerator beam intensity, so the calculation of the collection efficiency is of the utmost importance. The Monte Carlo simulation is one of the important methods for simulating the particle transport process and analyzing the energy deposition. A Faraday cup to be installed around the analysis magnet AM2 in the linac of BEPCII, was simulated using the Monte Carlo program, FLUKA. The curves of collection efficiency vs the different parts of Faraday cup structure wereobtained, the energy deposition distribution and the charged particle distribution of the incident electrons were also simulated, and finally the optimization designed size of the Faraday cup was given.