Positive flow response characteristic in vertical tube furnace of supercritical once-through boiler provides a feasible and effective way to alleviate the thermal maldistribution to some extent. In the present paper, theoretical derivation and simulations were performed to investigate the mechanism and forming conditions of positive flow response characteristic systematically. Two types of transition criterions were built up to judge positive or negative flow response characteristic. Parametric study was carried out to analyze the influencing factors. Both the operating conditions (mass flux, pressure, temperature or quality, heat flux) and the geometric parameters (inside diameter, length, pitch and type of tube) were investigated. In addition, thermal-hydraulic simulation of a 600 MW supercritical W-flame boiler has been carried out to take all these factors into account simultaneously. The conclusions drawn from this study will be of help to the design of large capacity supercritical once-through boilers as well as the refurbishment of existing supercritical or subcritical boilers.