A general analysis for the first-order design of the twoconjugate zoom system, which consists of three lenses and has a real or virtual image, is presented. The design formulas are derived. Of twoconjugate zoom systems, we analyze the solution areas in the system parameter diagrams under two particular initial conditions in which the object/image and pupil magnifications of the middle lens are taken to be 1 and 21 or 21 and 1. Two design methods are proposed. Several examples are given to demonstrate the proposed design procedures.