Scanning slit is an important element for controlling exposure dose in step-and-scan lithography. Too large a penumbra of the blades edge in scanning slit could affect the exposure performance. Firstly, according to the illumination principle of a step-and-scan lithography, the calculation formula of the penumbra of the blades edge at the reticle plane was derived by analyzing the relationship between the intensity distribution of the reticle and the thickness and location of the blades in the scanning slit. The theoretical value of the penumbra of the coplanar and noncoplanar blades edges at the reticle plane was verified using the simulating light model of a 0.75 NA lithography. Secondly, a high-accuracy scanning slit apparatus with four coplanar blades was developed, which not only meets the synchronization performance requirements of the step-and-scan lithography, but also reduces the penumbra of the blades edges in the X and Y-direction. Finally, the dynamic performance of the scanning blades and the actual penumbra of the blades edges at the reticle plane were also experimentally tested. The test results indicate that when the scanning speed reaches 470 mm/s, the dynamic position error of the blade is within ±30 μm and the penumbra of all edges does not exceed 0.5 mm and meets the requirements of the 90 nm step-and-scan lithography.