In the optical network, a filter with flat-top response can complete the quick and accurate channel optical detection with wavelength. With two Fabry-Pérot cavities coupled in series, a double half-wave filter with flat-top transmission characteristic is demonstrated. The effect of the random growth error on the transmission performance of the double half-wave filter with flat-top response is studied, with the transmission matrix method. The simulation indicates that there appear two peaks with different height in the transmission spectra when the physical thickness difference of the two Fabry-Pérot cavities exceeds 1 nm, which illustrates the two-peak asymmetry in the transmission spectra of the tested devices. In addition, the concept of interface random undulation is used to explain why the measured bandwidth of the filter is larger than the theoretical result.