Abstract: The lightweight structure design of aluminum mirrors is a difficult problem in the complex optical mechanical system with multi-channel detection, large number of mirrors, strict requirements for volume and quality, cross folding of optical paths in space, short distance of optical components, and easy interference. Either traditional methods have high lightweight rate, but the process is complex, or the lightweight rate is limited. For solving these problems, a new structural design method of special-shaped lightweight aluminum reflectors for space spectrometer is proposed, which includes material selection, configuration design, assembly deformation unloading, lightweight design method, surface coating, detection and error compensation, The experimental verification of a space spectrometer shows that the lightweight rate of all aluminum mirrors designed by this method is better than 50%. The development and test of a space spectrometer show that all aluminum mirrors designed by this method can meet the requirements of the system, and the lightweight rate is better than 50%.This method can be applied to various types of space opto-mechanical systems involving aluminum mirrors, and can provide reference for aluminum mirror design and compact opto-mechanical system design.