Collecting airborne spores of rice blast in rice fields using spore trap devices has currently become an important approach for devising strategies early and effectively controlling rice blast. In order to solve the problems of large volume and high cost of existing spore detection system, a spore detection method for crop fungal diseases was proposed based on diffraction reconstruction technology. Based on Huygens-Fresnel principle and angular spectrum theory, a spore detection system for fungal diseases, including enrichment and sampling mechanism, was designed by using diffraction imaging complex reconstruction calculation. The system can complete a series of operations such as enrichment, sampling, shooting, reconstructing and detection, and reconstruct the original spore image of fungal diseases by reconstruction algorithm. According to the morphological characteristics of reconstructed images, two important parameters, area and thinness ratio, were extracted to detect and identify spores. Rice blast spores were selected as the research object to carry out detection and verification experiments. The experimental results showed that the system could capture the micro-images of diffractions of rice blast spores, with 2592 pixels×1944 pixels resolution. The experiments validated that the correlation coefficient between the detection results of rice blast spores and the identification results of artificial microscope can reach 0.99, while the average detection error rate was 5.91%, which had good accuracy. The research provided a design scheme for the research and development of low-cost early warning equipment for crop fungal diseases. 