Abstract: The heat load between the heat transfer tubes of once through steam generator (OTSG) is unbalanced, which is not only nonuniform along the tube length, but also is biased between the heat transfer tubes. In order to master the flow instability of OTSG under the actual working states, the flow instability of parallel two channels for nonuniform and asymmetric heating conditions was simulated by Relap5 program. The total heating power of uniform heating channel and nonuniform heating channel was equal, but the heat flux of nonuniform channel was divided into three sections along the channel to simulate three phase zone of OTSG: subcooled water regime, twophase mixture regime, and superheated steam regime. The heat fluxes of them are 300, 430 and 350 kW/m2, respectively. For the asymmetric heating condition, the heating power between the two channels is not equal, but the sum of the heating power of the two channels is equal to the sum of symmetric heating channel. The axial heat flux of asymmetric heating channels and symmetric heating channels is uniform. The results show that the mass flow in the parallel two channels is equal for the axial nonuniform heating condition, the oscillation frequency is the same and the oscillation phase is opposite. Because the heat flux of the inlet section for nonuniform heating condition is lower than that of uniform heating condition, the position of nucleate boiling started in the channel is more backward, the corresponding singlephase length is longer, so the stability of the system is better. At higher subcooling, the stability of nonuniform heating system is better than that of uniform heating system. With the decrease of subcooling, the stability difference between them decreases gradually. For the asymmetric heating condition of parallel two channels, the mass flow distribution in the two channels is no longer same, but the oscillation frequency in the two channels is same and the phase is still opposite. The mass flow in the channel with low heat flux is higher than that of the channel with high heat flux, and the wall temperature amplitude at the outlet is smaller. The stability of symmetric heating system is better than that of asymmetric heating system. With the increase of asymmetry degree, the imbalance of mass flow distribution in the two channels will be aggravated, the stability of the asymmetry heating system will deteriorated gradually.