The invention discloses a saturation flow rate calculating method for an urban road intersection. The method includes collecting traffic data of the urban road intersection; performing pre-treatment on the traffic data and storing result data obtained through the pre-treatment; performing real time calculation according to a Grubbs algorithm and storing average time headway of the urban road intersection in single cycle; according to a K-means clustering algorithm, performing clustering analysis on the average time headway of the urban road intersection in single cycle in identical weather conditions and acquiring a clustering analysis result; calculating the mean value of time headway in a saturation condition and acquiring the saturation flow rate of the urban road intersection in different weather conditions. The method provided by the invention improves precision of the saturation flow rate of the urban road intersection in different weather conditions.