The properties of quantum mechanics and its difficulties and problems are discussed, the reasons resulting in these diffuculties and direction of development of quantum mechanics are carefully studied. In practice, quantum mechanics neglects the real motions of the microscopic particles and backgound field as well as the real interactions between them. Therefore it can describe appromately the states of microsco[ic particles. For solving these difficulties of quantum mechanics, we proposed and established a nonlinear quantum mechanics based on the properties of macroscopic quantum effect and theories of superconductivity and superfluidity. In the new theory the elementary principles are proposed, its properties are also investigated. Subsequently, the motion laws and some main properties of microscopic particles in nonlinear quantum systems, includong the wave-particle duality of the solution of the nonlinear Schrodinger equation, the stability of microscopic particles described by nonlinear quantum mechanics and the classical rule of microscopic particle motion, are studied using these elementary principles and theories. We obtained a lot of new and interesting results from this investigation, for example, the microscopic particles have really a wave-corpuscle duality in nonlinear quantum mechabics. The microscopic particles are localized due to the nonlinear interaction, have a determinant size, mass, energy and momentum. Thus the microscopic particles have not only a corpuscle but also a wave features, which are completely different from those in quantum mechanics. Hence we conclude that nolinear quantum mechanics is a new physical theory, a correct representation of describing microscopic particles and a necessary result of the development of quantum mechanics, but traditional quantum mechanics is only its an approximation and especial case at the nonlinear interaction to be equal to zero, it can solve the difficulties and problems disputed for about a century by scientists in quantum mechanics field. In the meanwhile, we use the new theory of nonlinear quantum mechanics to study properties of the excitons and phonons in an organic crystry (acetanilide). The results obtained show that the excitons and phonons become as a soliton having a wave-corpuscle duality in this system. Thus the correctness and avialablity of nonlinear quantum mechanics are demontrated using these experiments. We can predict that it has extensive applications in physics, chemistry, biology and polymers, etc.. Finally we state how the difficulties and problems in quantum mechanics are solved by the new theory of nonlinear quantum mechanics. Thus we can affirm from these investigations that the nonlinear quantum mechanics established by us is correct, quantum mechanics is only an appromate theory.