To reduce the calculation errors of ultrasonic flowmeters using the traditional time-difference method under different flow states, a correction method for flow velocity calculation is proposed. The flow field is classified into three states according to the Reynolds number (Re):laminar flow (Re<2 000), transitional flow (2 0004 000). Correction factors are added to the linear velocity distribution formula of the ideal flow state to address errors caused by different flow states. A water circulation system and a PIV system are used to collect flow rate data at Re=2 000 and Re=4 000 as calibration values. The linear velocity distribution formula with correction factors is combined with the integral time-difference method to calculate flow velocity and rate. By adjusting the correction factors and minimizing the errors between calculated and calibrated values, the correction factors for laminar and turbulent linear velocity distributions are determined as 1.847 1 and 1.436 8, respectively. Then, linear interpolation is applied to obtain the linear velocity distribution and corresponding flow values for transitional flow, The experimental data, are used for validation. Results show that the calculation error for transitional flow (Re 2 000-4 000) is about 0.2% relative to the experimental error, and for high-Re turbulent flow, the relative error is around 0.45%. This proves that the method of combining correction factors with the integral time-difference method via Reynolds number classification is effective and yields more accurate results.