本文讨论了蒸汽射流中被抽气体分子浓度和蒸汽流压力的变化情况。根据斐克第二定律的扩散微分方程式，导出了油蒸汽流泵的抽速计算公式，进而求得了第一级喷嘴直径计算式。用文中的公式对几种油扩散泵的抽速和第一级喷嘴直径等参数进行了验证计算，其结果与这些泵的实测值或实际使用值接近或相同。 This paper discusses the changes of gas molecules concentration and vapor pressure in the steam jet. According to the diffusion differential equation of Fick’s second law, the calculation formula of pumping speed of oil vapor flow pump was derived, and the formula of the first-stage nozzle diameter was obtained. The parameters of pumping speed of the oil diffusion pump and the diameter of the first stage nozzle are validated and calculated by the formulas in the text, and the results are close to or the same with the actually measured or actual values ​​of these pumps.