Under the effectivemass approximation,the analytic expression for Selffocusing effect in ZnS/CdSe/ZnS coreshellshell spherical quantum dots was derived by using the density matrix method. The numerical caculations were perforomed on the thirdorder susceptibilities as a function of the size of quantum dots and the frequency of the incident light were analyzed.The numerical results reveal that the thirdorder susceptibilities will increase with the increase of the quantum dot size, and position of the peak will red shifts. The conclusions provide a theoretical basis for experimental research and practical applications, and for research and improvement of photoelectric device with references.