Abstract: To calculate casing ratio continuously and instantaneously in tobacco primary processing line, and meet the requirements of high casing uniformity, the principle of casing, the effects of unit time value and stack delay on the calculation of instantaneous casing ratio were investigated basing on the statistical analysis of the data from a number of batches, and a reasonable method for calculating instantaneous casing ratio was proposed. The results of application showed that the method could intuitively assess the uniformity of casing process and timely monitor and alarm the casing problems, such as case missing or ill proportioning. The stability of casing system was effectively improved, the process capability index (C_pk) increased from 1.41 to 1.54. The assessment of casing stability was realized through off-line statistical analysis, which formed quantitative index and provided a reference for quality evaluation.