Abstract: Neutron total reflection is the principle of the neutron guide, which can transmit neutrons to the place far away from the source with very small neuron loss. In the design and application of neutron guides, it is very important to simulate neutrons transmission by Monte-Carlo method. A neutron guide system including three neutron beams at a reactor is under construction now. A Fortran code was developed to simulate cold neutrons transmission in the guides, and the neutron fluence rate and the spectra at each exit of the guides were gained. These data were the key input for the design of all spectrometers which would be installed along the guides.