A numerical simulation is performed by adopting unsteady mathematical mode for the confined circular cylinder flow and heat transfer in a sudden expansion channel. The simulations were carried out under different Reynolds number Re and the sudden expansion ratio of the channel Er. The results indicated that numerical solutions could be steady state, periodical oscillatory or chaotic with the change of Reynolds number. Meanwhile, the heat transfer around the cylinder was enhanced obviously with the increase of Reynolds number. The heat transfer around the cylinder was weak with the increase of the channel expansion ratio when 10≤Re≤100 or 250 ≤Re≤300, but when 100＜Re≤200, heat transfer around the cylinder did not always decrease monotonously.