By comparing of the research and development of optical system in a large telescope at home and abroad,a scheme for establishing error budge system was presented to determine the influential factors on optical system to ensure the expected precision in design,manufacturing and assembling of large aperture optical system. By taking a 1.2 m telescope optical system as an example, the conformation of error budget was introduced and a reasonable static error allocation was worked out. Firstly,a total error standard was determined, then, the number of error budgets was counted,finally,error budge result was given based practical ability of manufacture and assemblage.Experimental results show that the RMS wavefront error of this telescope optical system is less than λ/8.5, which matches the machining and assembling capacity.