Angular multiplexing excimer master oscillator power-amplifier laser (MOPA) system is simplified by object-image conjugate structure. Relations between the drifts of optical components and the deviation of corresponding imaging plane are derived by transfer matrix and ray tracing. The index weight is determined by improved analytic hierarchy process according to nine scale evaluation matrix, and the beam positioning stability requirement of optical components which affects system stability is acquired. Results show that the error budget values of optical components is related to the focus of image-relaying and the distance from object plane. The values change conversely with the focuses. The optical components stability in laboratory environment is measured and compared with those of error budget.