The incompatible numerical manifold method (INMM) is developed on the basis of numerical manifold method (NMM). The advantages of INMM are that the calculation accuracy and computing efficiency can be greatly increased without adding generalized degrees of freedom. The expressions of element strain matrix and element stiffness matrix are given based on eliminating the internal parameters. On the basis of least potential energy theory the stability and convergence are analyzed and discussed in Hilbert space, and the basic condition ensuring uniqueness and convergence of solution is given. The theorization of INMM is perfected. To illustrate the stability and convergence of the present approach, numerical examples are provided. It is shown that this method produces highly accurate and convergent results.