Abstract: An objective lens, an optical imaging device, an optical system, and a test method of the optical system, relating to the field of optical testing. The purpose is to optimize the structure of the objective lens. The objective lens comprises a housing and an optical focusing assembly. The housing comprises a cavity, and the cavity is provided with a first opening and a second opening. The optical focusing assembly is mounted in the cavity, the focal point of the optical focusing assembly is located at the second opening, and the optical focusing assembly is configured to focus light entering the housing from the first opening a to the focal point. The optical focusing assembly in the objective lens focuses light on the basis of one of the reflection principle and the refraction principle.