Abstract: A continuous zoom optical system with working focal length of 252 mm~504 mm was designed based on the basic principle of periscope.Instead of traditional flip zooming which is a discontinuous zoom mode, the system adopts the mechanical-compensated optical configuration in which the given variable lenses groups move along the optical axis driven by a cam to achieve the continuous change of effective focal length (EFL). Through simulating and imaging quality analysis, it is proved that all the performance indicators meet the design requirements. Test results show that spatial resolving ability of the optical system is below 2′.