Aiming at the application problem of non-uniform heating on the supercritical boiler, a numerical investigation of non-uniform heating upward circular tube is presented in this paper. The turbulent model k-epsilon (RNG) with enhanced wall treatment is employed after validating against the classic experimental results. Afterwards, the non-uniform heat transfer characteristics and mechanism, the influence factors and the criteria for onset of buoyancy effect on heat transfer of semi-heated tube are analyzed. The simulation results indicate that: (1) for non-uniform heating tube, the temperature distribution of cross section shows a great non-uniformity along the circumference, the heat transfer enhancement only appears in part area of cross section; (2) the key factors such as heated mode, heat flux, constant G/q and pressure are all influential on the thermal performance of non-uniform heating tube; and (3) investigate a good criterion standing out from three criterions to describe the onset of the secondary flow in non-uniform heating tube. (C) 2019 Elsevier Ltd. All rights reserved.