We propose an on-chip optical flat-top filter based on silicon-on-insulator with low ripple factor and high shape factor. Our scheme is based on a racetrack microring resonator-assisted asymmetric Mach-Zehnder interferometer, and we realize a filter with the ripple factor (the ratio of the maximum power to the minimum power in passband) of 0.11 dB, the shape factor (the ratio of 3 dB bandwidth to 20 dB bandwidth ) of 0.79, the sidelobe suppression ratio of 22.78 dB, insertion loss of 0.22 dB, and the free spectral range (FSR) of 5.22 nm. The FSR can be tuned by reducing the lengths of relevant waveguides proportionally without any changes in the structure. When the size is 0.58 of the original one, the FSR increases to 11.17 nm, and the shape factor is 0.81. The insertion loss is 0.21 dB, and the ripple factor and sidelobe suppression ratio are about 0.14 dB and 16.83 dB, respectively. Finally, we simulate the effect of the fabrication error on the performance of the smaller filter, and further verify that we can eliminate the effect of the fabrication error by embedding micro-heaters in the filter. The flat-top filter has advantages of small size, light weight, low fabrication process, large fabrication tolerance, low ripple factor, high shape factor, and low loss. It can be widely used in frequency combs generation, optical switch and some other optical signal processing fields.