Abstract: The Leaf Wetness Duration (LWD) is a key input factor of disease warning systems for crops in solar greenhouse. The leaf wetness sensors could automatically monitor LWD in real time. Since the interactions between microclimate and crops influence LWD, leaf wetness sensors should be calibrated in solar greenhouse conditions. In this study, the calibration method of leaf wetness sensor was developed with mini cucumber at fruit harvesting stage, and the experiment was conducted in the solar greenhouse during late summer and early autumn. The leaf wetness sensors were oriented at a 45° angle, and firstly the sensors was misted with water from a pipette, and then they were attached to the wet leaves. In these two different conditions, the dry/wetness threshold was determined. The monitoring results of different sensor deployments were compared, which included attaching to leaf margins, tips and lower surface and placing below the leave. The effects of rainy or non-rainy conditions on the measurement results were also investigated. The results showed that the dry/wetness threshold of “6” was obtained by attaching the sensors to the wet leaves, in this case, the monitoring effects were good and the errors were around 1 h; when the sensors were attached to leaf margins and tips, the monitoring accuracy (fraction of correct estimates) was around 0.75-0.83; furthermore, the monitoring effects in non-rainy conditions were better than rainy conditions. The results indicate that the calibration method for leaf wetness sensors can be used for monitoring leaf wetness duration of cucumber and meet the requirement of cucumber disease early warning system in solar greenhouse.