Abstract: The leaf wetness duration (LWD) is a key input factor of disease warning systems for crops in solar greenhouses. The LWD estimation model based on canopy relative humidity (RH threshold model) is one of the easiest models. To calibrate and validate the model in real greenhouse conditions, the experiment was conducted in the solar greenhouse during late summer and early autumn with mini cucumber at fruit harvesting stage. The relative humidity (RH) data were obtained automatically at every five-minute interval. The trial-and-error method, average value method and the method based on frequency of leaf wetness were used to calibrate the RH threshold, and then RH thresholds were obtained including 90%, 89% and 93%, respectively. In addition, the calibration results were validated by root mean square error (RMSE), regression analysis and a series of error analysis indicators. The results showed that the estimation effects of trial-and-error approach and average value approach were better than the method based on frequency of leaf wetness, and the errors were around 1–2 h. Compared with the leaf wetness duration that was over 3 h, the monitoring effects of RH threshold model were acceptable. However, from the validation results, the estimation effects of average value approach were better than trial-and-error approach, which indicated that it was impossible to apply only one calibration method in practice. The calibration and validation methods and the estimation model of leaf wetness duration based on canopy relative humidity can be used for monitoring leaf wetness duration of cucumbers and meet the requirement of cucumber disease early warning systems in solar greenhouses.