Abstract: The flow instability in the parallel channels under different radial heat distribution was investigated in the following condition: the system pressure is 0.89~1.32 MPa, the mass flux is 500-750 kg/(m2?s) and the inlet temperature is 58.5℃~132.3℃ with the upflow deionized water. The test section consists of 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. The results show that the distribution of the radial heating heat flux has no obvious effect on the unstable boundary of the parallel multi-channel flow, when dealing with the parameters of unsteady flow in the hot channel. For the parallel multi-channel structure with different heat flux distribution, the flow instability boundary is the same as that of the parallel multi-channel structure with complete symmetric heating.