The Newtonian fluid flow in a rectangular channel with sudden expansion followed by sudden contraction was simulated numerically for the whole flow region.The nonlinear characteristics and temperature field were investigated with various Reynolds numbers and geometrical dimensions.The results show that the temperature field will evolve from symmetric state to asymmetric state with the increase of Re.When Re<Re_c(critical Reynolds number),the symmetric state is stable.However,when Re≥Re_c,the flow loses stability and develops from symmetric state to asymmetric state via a symmetry-breaking bifurcation.The expansion ratio and aspect ratio can affect the value of the critical Reynolds number.The analysis results show that when the Re number is greater than the critical Re,Nu number will change significantly,and then affect the heat transfer of the pipeline.