Femtosecond laser filamentation propagating tens of meters to several kilometers with high intensity in the atmosphere has been demonstrated as a powerful tool for remote sensing. In contrast to the refractive systems, the reflective optical systems possess a variety of advantages including broad bandwidth, large aperture, light weight and low energy loss. However, astigmatic aberration is inevitably introduced by off-axis reflective mirrors. It can greatly affect the filament quality, which is critical for exciting and detecting the fluorescence of target molecules. Here we elaborately design a free-form phase plate to correct the astigmatism in off-axis reflective optical systems. It is demonstrated that the free-form surface exhibits excellent performance, significantly reducing the astigmatic difference from 44 cm to 4 cm and increasing the maximum acoustic intensity by a factor of 53. In addition, extremely strong nitrogen fluorescence spectra have been detected. These results indicate that the free-form phase plate can effectively compensate for astigmatic aberration in off-axis reflective system, providing a guiding significance for the optimal control of filamentation and remote sensing.