The hydrodynamic modeling method in gas-stirred ladle system that widely used to measure the mixing time was reconsidered and discussed in this paper. The effect of injected salt tracer amount on the mixing time measurement was investigated. For the mixing time curve, the final concentration increased with the tracer amount increased; also this curve exhibited occasional sharp increasing tendency when a larger amount of tracer was injected. Besides, the obtained mixing time decreased with the tracer amount increased. The error of mixing time measurement was mainly from the tracer concentration fluctuation beyond the criterion range when a much smaller amount of tracer was injected. While the criterion range increased with the tracer amount increased for a certain criterion of mixing. Finally, the tracer selection should be considered combined with the criterion of the definition of mixing. In the present study, the dimensionless tracer amount (defined as the ratio of tracer amount to the water volume in hydrodynamic model) of KCl solution should be larger than 0.2692 × 10−3 or 0.6731 × 10−3 when 5 or 1% criterion was used.