Abstract: Distant extrapolation is usually used during the start-up of the research reactor, by lifting the control rods step by step to reach the critical state. Due to the non-linearity of the integral worth of the control rods, this process was risky or conservative, especially when the rods were positioned in the nonlinear region. A formula could be derived from the point reactor model, in which the reciprocal of the count rate was proportional to Δk_eff. Together with the integral worth curve of the control rods, the effect of the non-linearity could be corrected. This method was valideted by critical extrapolation data.