Abstract: In order to reveal the special application process of probability in handling practical financial problems, the author independently deduces the steps to find option pricing formula through probability approaches on the basis of the hypothesis of classical financial principle. First through unassailable logic deduction, the detailed process of transforming practical financial background into probability model is given, which includes the random process of stock price fluctuation and the establishment of mathematic model to describe the equilibrium option price change of no arbitrage by Itô; Second， the substitutional formula for integral variables in finding option pricing formula by probability model is presented.