Abstract: This paper is concerned with the theory and algorithm of complicated unstable deployable space trusses. Taking the Cartesian coordinates of ideal truss joint as unknown variables, the geometric constraint equations for complicated unstable deployable space truss are derived. The essential characteristics for quasi-static motion are illustrated. The stabilization condition and the object function are introduced and the properties of the structure at limit stable state are highlighted. The influence of the quadratic terms of the geometric constraints is also discussed. Numerical examples are provided to verify the present method and algorithm.