The St. Petersburg Paradox, an important topic in probability theory, has not been solved in the last 280 years. Since Nicolaus Bernoulli proposed the St. Petersburg Paradox in 1738, many people had tried to solve it and had proposed various explanations, but all were not satisfactory. In this paper we propose a new pricing theory with several rules, which incidentally resolves this paradox. The new pricing theory states that so-called fair (reasonable) pricing should be judged by the seller and the buyer independently. Reasonable pricing for the seller may not be appropriate for the buyer. The seller cares about costs, while the buyer is concerned about the realistic prospect of returns.The pricing theory we proposed can be applied to financial markets to solve the confusion that financial asset return with fat tails distribution will cause the option pricing formula to fail, thus making up the theoretical defects of quantitative financial pricing theory.