To analyze the influence of misaligned parameters of circular aperture and optical component on propagation properties of flattened Gaussian beam when flattened Gaussian beams passing through an optical system, an approximate analytical expression of the output field distribution for flattened Gaussian beam passing through an misaligned optical system with misaligned circular aperture is derived using approximate expanded formula of misaligned circular aperture function and generalized diffraction formula appropriate for misaligned optical system. The relations of output field distribution to the parameters of flattened Gaussian beam, the aperture size and various misaligned parameters of aperture and optical element, etc. are given. Some influences of various misaligned parameters on output field distribution are analyzed quantificationally for given optical system. Result shows that various misaligned components effect on the intensity distribution of output beam. But the influence of misaligned lens is more obvious than that of misaligned aperture when misaligned parameters value is small.