The research of fatigue crack threshold is an issue in fatigue research. Though there are many research methods, calculation formulae and test data, most of the formulae are evaluated by experiments and have test constants in them, which lead to inconvenient application. Based on displacement fields around crack tip and fatigue theory, A formula that can describe fatigue crack threshold related to material properties is proposed. Each parameters in the formula has definite physical meaning. And it is convenient to use this formula. Using this formula, the material threshold is primarily calculated before design. At last, using existing data to verify, it is shown that the formula is reliable.