Freeform illumination is one of the necessary techniques in 28 nm technology nodes and beyond. The micromirror array (MMA) has been widely used in lithography freeform illumination systems due to its programmability and high free degree. The MMA allocation algorithm is the key to generate the target freeform illumination source. Its computational speed and precision affect the generation speed and precision of the target illumination source as well as the process window size of the generated illumination pupil directly. In this paper, an MMA allocation method based on deconvolution is proposed. The target freeform illumination source can be obtained directly with the deconvolution and quantization processes. Without the iterative optimization process, the computational speed of the proposed method is much faster than that of the traditional method. The numerical simulation results show that the difference between the target source and the MMA source generated using the proposed method is less than 0.2%. Compared with the process window loss of the target source, the process window loss of the MMA source generated by the proposed deconvolution method is less than 0.5%. Compared with the traditional allocation method, the runtime of the proposed method is less than 0.05 s and has improved by 1463 times.