Infinitely narrow photon beams have been used in the former Monte Carlo modelling about the signal of optical coherence tomography (OCT). However, the focused spot size is finite and determined by the center wavelength of the optical source. In the present paper, finite-size photon beams are launched into a multi-layer numerical phantom, and the signals of OCT are obtained based on Monte Carlo technique and Mie theory. By use of the model we confirm the shower curtain effect as researchers have found before.