The invention provides a theoretical calculation method for the lean combustion limit of fuel. The method specifically includes the following steps that firstly, the mole fraction of each component is determined; secondly, each type of diluent gas is converted to a mole fraction of equivalent N2; thirdly, mixed gas containing fuel is approximated to mixed gas composed of fuel, O2 and N2; fourthly, normalization processing is conducted on the mole fractions of the components; fifthly, the mole fraction ratio lambda of N2 to O2 is worked out; sixthly, a non-theoretical specific general reaction equation of fuel, O2 and N2 under the oxygen-enriched condition is constructed; seventhly, the adiabatic combustion temperature of fuel under the lean combustion limit is assumed, and the mole fraction xf of fuel is obtained through reverse deduction according to an adiabatic combustion temperature calculation formula and the non-theoretical specific general reaction equation.