The present invention provides a mixed crystalline form VIII of agomelatine, its method of preparation, application and pharmaceutical composition. The said mixed crystal consists mainly of crystalline form VI of agomelatine. The said mixed crystalline form is stable and has good reproducibility. Through stability tests, it has been found to be superior to the crystalline form VI in terms of stability. As a result, the crystalline form VIII of the present invention possesses advantages in pharmaceutical preparation.