Abstract: This paper expounds the concept of critical flow rate in horizontal well, andmakes evaluation and analysis on some calculation formulas of critical flow rate at home andabroad, and point out the problems in Chaperon formula, which is in common use at the moment. This paper makes a modification for Chaperon formula, and derives the generalizedprediction formula of critical flow rate for horizontal well. Field applications show that themodified Chaperon formula and the generalized prediction formula can be used practically.