To get images of high quality with aerial camera,the focus of the lens and the situation of the real image plane must accord with the technical standards. A system is put forward which can automatically measure focus and estimate whether the aerial camera is off-focus and the situation of the real image plane is in the focus plane range or not. The work principle,the hardware composition and the software design are discussed in detail. The system acquires examining-focus information with collimator,receives images information with CCD camera which can be regarded as optoelectric conversion apparatus,processes data with the center of gravity method,and realizes the axial flow of the CCD camera translation with the grating displacement sensor and the digital readout together. Then,the optimum image situation and the image size can be fixed on. The optimum image situation can be used to judge whether the camera is off-focus,and the image size can be used to calculate the focus. The accurate rate of examining focus can reach almost 99.87% and the estimation of off-focus or not accords with experiment result of flight.