The UAV passive positioning system can realize the bistatic passive positioning of the target with the help of radio and television signals, and the performance of the signal processing board directly determines the positioning accuracy. In order to adapt to the light UAV platform and ensure the accuracy of positioning measurement, a miniaturized signal processing board is designed in this paper. Firstly, the traditional architecture of the signal processing board is simplified and optimized, and FPGA+AD9467 is used as the overall architecture of signal processing. In order to solve the signal integrity problem caused by the miniaturized design, the Cadence and HFFS simulation software are used to simulate and analyze the signal integrity of the processing board during the whole design process, and the signal integrity of the processing board is ensured from the aspects of reflection, crosstalk, and electromagnetic shielding. In order to improve the signal resolution ability of the processing board, a four-spectrum line difference algorithm based on Blackman window function was designed on the basis of the traditional FFT spectrum measurement. The algorithm is used to test the spectral characteristics of the received signal of the processing board, which reduces the measurement error caused by spectrum leakage and fence effect and improves the measurement accuracy. The SNR is better than 90 dB and the SFDR is better than 75 dB in the 29.5 kHz bandwidth. The test results show that the signal processing ability of the designed miniaturized signal processing board is excellent, which meets the application scenarios of passive positioning of light UAV.