For a reliable diagnosis of plant diseases and insect pests,spectroscopy analysis technique and mutispectral imaging technique are proposed to diagnose five cucumber diseases,namely Trichothecium roseum,Sphaerotheca fuliginea,Cladosporium cucumerinum,Corynespora cassiicola and Pseudoperonospora cubensis. In the experiment,the cucumbers’ multispectral images of 14 visible lights channels,near infrared channel and panchromatic channel were captured using narrow-band multispectral imaging system under standard observation environment.And the 5 cucumber diseases,healthy leaves and reference white were classified using their multispectral information,the distance,angle and relativity.The discrimination of Trichothecium roseum,Sphaerotheca fuliginea,Cladosporium cucumerinum, and reference white was 100%,and that of Pseudoperonospora cubensis and healthy leaves was 80% and 93.33% respectively.The mean correct discrimination of diseases was 81.90% when the distance and relativity were used together.The result shows that the method realized good accuracy in the cucumber diseases diagnosis.