Abstract: The flow instability in the multi-parallel channels was investigated under the condition of: P=0.89~1.32 MPa, G=500~750 kg/ (m2?s), and ΔTsub,in=58.5℃~132.3℃ with the upflow deionized water. The test section consists of 2, 3 and 5 parallel tube channels with the dimension of 1400 mm×Φ8 mm×2 mm. The characteristics of the flow instability in the parallel channels and the flow instability boundary was studied based on the experimental results. It shows that the flow excursion appears when the water in the channels becomes two phase, and period oscillation appears when the mass quality at the exit of the channel reaches to a certain level. The number of the heated channels has little effect on the flow instability boundary.