The Jiangmen Underground Neutrino Observatory (JUNO) is a multi-purpose neutrino experiment with a 20 kton liquid scintillator detector, which is under civil construction. Its primary goal is to determine the neutrino mass ordering by precisely measuring the fine oscillation pattern of reactor antineutrinos emitted by nuclear reactors at the baseline of about 53 km. To help design the detector and predict its performance, a Geant4-based full detector simulation for JUNO has been built under the SNiPER framework. In this poster we will describe the implementation of the Monte Carlo Simulation and present expected detector performance such as the energy resolution.