Abstract: According to fluid mechanics, molecular diffusion and heat conduction, the paper offers a new theoretical analysis for determining the drain pipe location and its scales at a model-made crucible. This is very important in getting optical glass liquid of good quality. By calculating the glass liquid flow of a model-made crucible, we can determine the location of the drain pipe at the bottom of the crucible for eliminating "stripes" in the products.By computing the temperature field in the drain pipe, the paper yields the relationship among the minimum value of the length Lm, diameter D, Reynolds number Re and Prandtl number Pr of the glass liquid and offers a foundation for determining the length of the drain pipe. A calculation example and a formula for determining the diameter D of the drain pipe are given in the paper.It is believed that the analysis and calculation given here are of some value to designing mixers in chemical engineering industry.